We are planning an addition to our home later this year which will include an 8'x10' concrete front porch. Full 9' basement in addition and under the porch so this will net me a finished interior dimension of around 7'x8' for a walk-in gun safe/vault. 8" poured concrete walls with a concrete roof.
Plan is to buy a Sturdy Safe vault door with fire insulation added to door.
Questions:
1. I'm considering pouring the vault room floor 4-6" higher than the rest of the floor to keep out water dumped in by the FD in the event of a fire. There will be a drain in the vault room to dump a dehumidifier into and to remove FD water that makes it in. Existing basement is a walkout, so as soon as the walkout door is compromised that should limit the amount of standing water from the FD. But the real goal is to keep all the water out since it may take weeks to get into the room after a fire and I don't want any water in there at all if possible. I realize it depends on the fire and how much water gets dumped in, but is a 4-6" higher floor even worth bothering with if this is the only purpose?

2. Hvac, how important is a fresh air exchange in this environment? About 2' of the foundation will be above grade and frost here can reach a couple feet some years, probably less here as it's sheltered. My plan is to 2x4 stud the walls and ceiling and then spray foam insulate it. Dehumidifier should provide some heat from motor and could easily add a very small electrical baseboard unit to maintain a decent temp. Hell a 100 watt bulb might do it. I'd be in and out of the room at least weekly most of the winter. Builder discussed a small mini-split but for simplicity, expense, and wall space in a room this small I'd like to avoid it. Tying it into existing HVAC seems like a fire/smoke issue.
For reference, I have a small uninsulated utility room very close to this area in the existing basement and have never seen frost or moisture on the concrete walls even where it is above grade. But it has a regular closet door on it so not airtight either, plus it has a warm ceiling with the house on top of it whereas this vault will not.  
Thanks

My Bud has a gun vault with similar features. He had it built by a company that specializes in vaults. It has Its own HVAC system with inlets and outlets designed to melt closed in the event of fire. It keeps a constant 70* temp and 50% humidity.

Photos, HVAC piping
/ducts under the raised floor from which I took the photos:
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Nice.
But seems overkill for the amount of guns.

So if you had 12 guns you would buy a 12 gun safe.  Then when you buy the 13th gun you buy a 13 gun safe and so on....  Maybe he has big plans for the size of his collection.  I mean I know we are all millionaires with super model wives that bought 100 guns on our first day of collecting but maybe he didnt......

Wow OP, you're really convinced you're going to have a fire in your house.

Uh, I asked one question about fire, is it worth raising the floor slightly to keep out water in the event of a fire or is that not realistically going to work??
House fires are actually pretty rare and my house is fairly new. But since a concrete room with dirt/air on 3 sides should actually survive most fires the biggest danger seems like water getting in and setting there for a month. Seems like it's worth asking about while still in the planning stage.

Burglary is far more likely and has actually already happened to me (I caught them in the act). The concrete and vault door should solve most rookie attempts anyway.

Second question about necessity for fresh air exchange has nothing to do with fire. My current basement is very dry (I live on top of a hill) so if it is spray foamed tight and has heat and a dehumidifier, is it absolutely necessary to have some form of air exchange system?

That's nice. Probably finished nicer than mine will be and it looks a little deeper front to back. The HVAC guy did mention that he is familiar with vents that close in a fire that are used between floors in commercial buildings but didn't really expand on cost or how well it would work in a small structure like this. I'm not convinced it's needed? My current basement is not damp or musty, but maybe this room will get a little stanky w/o fresh air movement?

Edit: Looking again I see what you mean by elevated floor. I assume the door is behind you there, so you walk in and step down into the room? Is this in a basement? Generally HVAC in a basement is in the ceiling. Do you know why they built it this way, with HVAC on the floor and the step down in construction?

I'll just say in the event of a fire you're going to have a hell of a lot more than 4" of water in your basement.

My vault isn't fireproof. I wasn't so worried about that as everything can be replaced. Mine simply is secure(ish) from snatch and grab attacks. Keep in mind it's pretty easy to break through 8" of concrete so you may want to consider additional measures. Mine is a combination of concrete, lath, hardwood, and nail embedded wood. Any power tool that is good at cutting through one sucks at another so it slows them down a lot more.

Mine is humidity controlled to be kept at 50% RH, and the HVAC keeps it right at 70 year round, although it may creep up to 72ish if the dehumidifier runs a bunch.

Yeah, they are going to dump more than 4" of water in the house. I was counting on it running out the basement door about as fast as they can dump it in. But unless the door fails or someone opens it the water will be 8" before it can pour out the door glass (assuming it breaks quickly). I'm 10 miles from the volunteer FD out in the country, if the house is seriously on fire I'm not going to be in any hurry to call it in. I'd rather rebuild from scratch and hope the vault survives.
 
Again, fire is not a huge concern for me but an elevated floor seems like a simple thing to do if it actually works. Might not be worth it though. If anyone has had success with that let me know.
What's going in the vault is nothing special but more than insurance will cover and the NFA stuff takes time. The point of the vault is never outgrowing a safe while keeping out run of the mill meth heads. Real fire protection is a bonus and sold the wife on the added expense so she can protect photo albums, etc.

So onto the HVAC part. If you are fairly confident that you won't have moisture seeping into the vault, is fresh air circulation needed and why? Lots of people put a gun safe in a garage (summer humidity, wet cars, etc) and do fine with just a goldenrod, so why wouldn't a small baseboard heater and a dehumidifier accomplish the same thing in an 50-60 sq ft insulated room? Just trying to understand it.

As long as moisture is kept under control stagnant air isn't a risk to the contents. It just might start to get stale and musty.

JPK, see edited questions above please.

Point of reference on the HVAC thing. 30 some years ago I spent new years eve helping my dad drill 87 bazillion holes through his concrete foundation to reach the ~5x5 room that was under his front porch. Very similar to the room I will have with exposed concrete ceiling and walls exposed to air/dirt on 2 sides. His garage was under the house and this room was in the back of the garage. He insulated it with 1" or 2" white styrofoam insulation glued to the concrete and no HVAC (garage was heated though), but it wasn't as tight as a vault. Had a hollow core closet door framed in. He used it to store tackle boxes and garden tools and I never noticed any rust or must issues. That house is 10 miles north of me.

I have a humid basement and run a ceiling fan all year in the main room to keep air moving so mold or mildew is held down. The surplus gun stocks stored in the room next to it have no fan and they will develop one or the other. Keeping air moving with a fan might be an additional thing for a vault. The one I toured didn't have an appealing air quality.

Regarding the super model wife comment, they're always bi so get 2.

My basement is dry but it can sometimes get a little stale in the summer when the stairwell is closed and we don't run the AC vents because it stays cool enough without them. I could easily run a small fan in the vault if needed. Might look further into tying it into the basement HVAC with fire baffles. I want to do it right but this isn't going to be an Instagram gun room with a whiskey bar, it's the size of a spare bathroom and I just want my guns secured and not rusty.  

I must have missed the super model wife comment? My wife is very pretty but she has made it clear that I'm only allowed to have one, go figure...

You probably don’t need to have HVAC in the vault, but I’d recommend a small duct in and a return out to allow for air circulation to prevent mold and mildew. The dehumidifier might be enough though.

If you do put in ducts, you want fire dampers- they aren’t that expensive; ~$300/ea last time I looked. They use a fusible link that closes off when it gets hot (link melts). It doesn’t keep out smoke though- that requires a smoke damper ($1000) but that requires a tie in to the fire alarm system to actuate.

If you raise the floor to drain water out you need to give it a way to get out. If you have a floor drain you don’t need to raise the floor. If your vault is under its own roof you may never get water in it so there may be no concern.

If you’re really worried about fire, look at residential sprinklers. May not make sense for an addition but is the smart thing for new construction. Only the head where the fire is goes off and suppresses much more quickly the the FD can get there.

I'm not very worried about fire, at least not for the house itself. Given my location I would not be in a hurry to call 911 because I'd rather have a total loss, an opinion shared by several firefighter friends. But eventually they are going to show up and squirt water on the foundation and I'd like to keep that out of my vault. It will have a floor drain so maybe thats all it needs but it seems like things would still get stanky in there quickly if I couldn't get into it fast.
A few days ago I talked to my local fire chief. He thought that 6" of elevated floor or lip in the doorway would likely keep water out, given the walkout basement and how slowly they could actually get water to my location. At my square footage that's about 7500 gallons of water to get to 6", not accounting for leakage.
A lip or elevated floor is easy to do at the construction faze, but given how rare house fires are I'm not sure its worth dealing with the step in. It's something I was interested in though.

The HVAC thing is more important to me. I really don't want to dick with it for a room this small and the addition is already getting rediculously expensive. But, I don't want to end up with a shitty room and it can't easily be done after construction. Still, it seems like a dehumidifier and products like DampRid should take care of any moisture problems but I don't have much experience with what happens in a small sealed off room like this.

@JPK do you know the brand of the HVAC unit?

No, but will inquire.

I’m not sure the HVAC unit or dehumidifier is as significant as the brand of the “melt to seal” fittings, which I will also inquire about.

Since smoke damage is also a significant issue, I would run it without.  Maybe an option would be to prep it for a minisplit unit?   Then, you could run dehumidifiers and a fan, but if you find it's not enough, you can still easily install the minisplit.  I've never installed one, but I would imagine prep work is limited to some pipe to fish everything to the exterior unit.