From what I understand that would be completely legal, but, what if the ATF says it is 25.99"? Is there precedent?

Why is someone checking? As you stated, it is long enough, but if you are being checked, there is going to be more to the story.

Personally I say no, because I don't like VFGs.

Edit: Poll added, voted yes because FBATFE

Of course. My 11.5" AR is an SBR, which is allowed to be less than 26" OAL, so there's nothing to see. That being said, I don't like VFGs, so I wouldn't actually do it. But I wouldn't be scared to.

THIS.

OP, you talking about an ar-15 pistol?

the law is black and white on this. If you are sure on your measuring skills, go for it!
I don't like VFG myself.

Yes a pistol. It wouldn't matter if it were an SBR. My 11.5" Adams Arms Tactical EVO upper, with Phase 5 Hex2 buffer tube meaures exactly 26". I'm not comfortable putting a VFG on it. Maybe I'm being too paranoid.

stack a few crush washers on the end to extend the flash hider.

Nope that doesn't work, you measure from the end of the butt/buffer tube to the end of the muzzle. Not flash hider unless pinned and welded.

Now, add that VFG to the front handguard and you get what? If it's 25.99" you get one set of regulations, over 26" and it's another. http://www.ar15.com/forums/t_3_122/458387_ATF-Rule-on-Adding-a-Vertical-Fore-Grip-to-a-Handgun--Specifically-the-AR15-Pistol-.html

The real issue is that its either a pistol, or not, which is the interpretation of the local LEO/Conservation Agent who stopped you, the local law about transportation, and then how they enforce it IAW the laws in affect at that time of the year. THAT is more important - along with what were you doing at the time to get stopped?

Bigger problems in life that need more attention.

I will always configure an AR pistol to be <26" overall length.
I WANT to be able to treat it as a pistol, the conceal-ability is key for me.

The problem with adding a VFG onto a pistol with an overall length greater than 26 inches is that it changes the classification according to federal law. It goes from a pistol (without VFG) to a firearm. The big thing to remember is that a >26" pistol with VFG can not be concealed. If you conceal it, the firearm then is considered an AOW and falls into NFA territory. State laws vary also and some won't allow concealed carry of a weapon classified as a firearm but will allow a >26" pistol to be concealed with a CCW permit.

My pistol with a 10.5 barrel and Kak Super brace buffer tube measures 26.5 inches from end of the tube to the muzzle (not counting flash hider). I had a VFG on it just to try and ended up putting a AFG on it to keep it's pistol classification. Under Missouri law I can have a pistol concealed (no matter how long) in my vehicle but I can't have a firearm or rifle concealed and loaded.