I checked out the Liberty Safe Co. vault door. $4000, YIKES! 2nd floor? I don't think so. The weight was alot. I don't remember the poundage but it would take some structural reinforcement on most homes.  Unless you have alot of stuff to protect you might be better off with a safe than a vault.Good luck.

I don't know what kind of gun collection you have but a vault door requires alot of work. The door is expensive and if you don't secure the rest of the room as secure as the door it is a wast. If you are a regular guy with a normal size collection 10 to 40 guns get a nice safe and put it in your bedroom instead of an amoire. You can caddy corner it get a beautiful finish on it and it is furniture grade. If you have more guns then that get a bigger safe. If you have some cash you can hire the delivery people to help you bring it up the stairs. The national security has the feature that you can remove the door to make it weigh less for manuvering it. I like national security cause the door opens a full 180 degrees. The fort knox does not only opens 90 degrees.Unless they changed it. It sounds like it is not a big deal but when the safe starts getting full you need to open that door all the way to get things in and out. good luck

A buddy of mine built a nice little vault in his basement.  He used the standard Jail Cell type Door from a Correctional Supply Co.  Its solid steel with Locking mech already installed.  Total cost of steel door, steel frame(filled with concrete) and 2 jail keys= $900.

Gloftoe-
from what you've said, I'm thinking about the idea of using a nice big furniture grade safe on the main floor, say, in a "TV" room, and then finding a way to make a false wall or entry into that nice big closet upstairs.
You could make the hidden entryway secure to an extent, and keep your home defense style firearms up there, even using it as a "safe room", with communications, etc.  I'd like a second way out in case of fire, though.
Tough call.

Vault doors are pricey I'd opt for a safe or two, but fire burns UP dude and a safe on second floor even if it's fireproof may not mean much after your guns are trashed by the fall through the floor.  Do you have a basement?  Good place for a safe bolted down. No basement what about a concrete vault poured outside in a corner or as an addition with an inside entrance via a vault door?  Concrete ceiling - roof over it, side it- presto it look's like the rest of the house and FEMA will help defray costs as a storm shelter!!

Look into it

Boomin

Get a decent large safe, stick it in the basement.  Put stuff in front to sort of conceal it if possible.  Use another small safe or lockbox in your bedroom for your personal protection gun.  If you have a fire, its best off in the basement.  Put it on the 2nd floor and it will still end up in the basement after a fire, albeit banged up from the 2 story drop.  Also consider where an intruder is going to look for valuables.  Most people tend to keep their most valuable possessions (cash, jewelry, guns)in the bedroom.  Thieves know this and make it one of the first places they look.  Unless they have reason to believe otherwise, they won't expect to find anything of much value in the basement and will often bypass it - especially considering basements often only have one way in and out and they don't want to box themselves in.

Really effective gun vaults are built as gun vaults, not as converted closets.  That means the walls and the overhead poured solid, with rebar tied in.  Vaults are becoming popular in new home construction, also serving as shelters during hurricanes and tornadoes.  As has been pointed out, an upstairs closet would not make a good vault.

Just a thought... How about filling the voids of the interior walls with additional 2x4s or concrete? Pour a 1 1/2" concrete floor up to the point the entry door closes. Cover the walls in drywall. The outer door should open to the hallway and be solid wood. An inner steel door could be added which opens into the room and sits 1 1/2" higher, on top of the concrete floor. The concrete floor will make the outer door kick resistant (if it opens to the outside). This should give you quite some time regarding fire and theft, plus it will blend well with your decor.