Asking an administrative agency for their opinion only tells you whether they'd prosecute you if you were the longest blade of grass when the mower came your way. It doesn't tell you whether it's legal.
Statutes and case law tell you whether it's legal. There's just a statute here and very little case law. (The 1993 case isn't precedent because it was dropped at the District Court level when the ATF lost.)
That said, if you don't have the money to fight the court battle, it's safer to stand only as tall as the other blades of grass by not putting any sort of even remotely vertical grip on your pistol.
IAALBNYL (I am a lawyer, but not your lawyer), but I think the proper way to resolve this issue would be to take your favorite MANUFACTURED pistol that has a rail and pay the ATF the tax to make an AOW by putting a forward vertical grip on it. Then sue them to get your tax money back.
If I recall correctly, that's how cases like the Thompson Contender case started. You do everything legally by jumping through the hoops the ATF sets up (getting the tax stamp) and then you ask for your money back in court. If the court rules in your favor, you get your $5 back and you also set a precedent that can be cited in other cases. (That precedent is only persuasive at the trial court level, but if the ATF appeals the loss and still loses, you've created binding case law.)
Now, the reason I capitalized the word "manufactured" above is because of the wording of the statute. It says that a pistol is a "weapon originally designed, made, and intended to fire a projectile [...] when held in one hand, and [...] a short stock designed to be gripped by one hand and at an angle to and extending below the line of the bore(s)."
The important words are "originally designed, made, and intended." A Glock 17 is originally designed, made, and intended to be held in one hand and has a short stock designed to be gripped by one hand (etc). (That's the one original pistol grip.) So when you add a vertical foregrip to the front rail, you're taking something that's a pistol as originally designed and adding a new feature. BUT, and here's the important part, even though it's now capable of being held with two hands, it was still ORIGINALLY designed, made and intended to be fired with one hand.
The opposite case would be if you built a pistol-type gun from scratch and designed it with a vertical grip. Imagine making your own semi-auto Sten with no stock and a vertical foregrip. That wouldn't be a "pistol" under the law because it wasn't originally designed, made and intended to be used with only one hand.
See the critical distinction? It's whether the person that made the original gun put one or two grips on it.
So now we just have to get someone to bankroll the test case by paying the AOW tax to put a VFG on their Glock and then suing the ATF to get their taxes back. Perhaps Magpul would be interested in something like that? I don't know, but they sure would sell more foregrips if they were.
Aaron
Edit: Sorry for the bad information: the Form 1 tax for making an AOW is $200. Transfer tax on an AOW on a Form 4 is $5. So if you're making an AOW out of a Glock, as I suggested, you'd be fighting for your $200 back, not just $5. Too bad really--I'm confident someone could win such a case and it'd be funnier if it was only $5 you were fighting over.