Personally, I don't see the need for a vertical grip for a handgun.
Neither do I except for maybe that select fire Glock.
They already make a shoulder stock for it, which plugs into the hole in the grip's backstrap. Never used it; not sure if it's effective.
Why on EARTH is it NFA to add a simple grip support to the front of a handgun? Are the BATFE types worried about how much more lethally accurate this would make a handgun? Puhlease. If you're legal to shoot someone, you're legal to shoot someone. And of course if you really wanted to do it effectively, you'd use a rifle, which of course can legally have stuff like that.
Just more proof that the laws pertaining to guns are absurd on their face, in 99.9% of cases.