Can I use a Vertical Foregrip on my AR pistol?

The ATF has issued an opinion letter that outlines when you can use a VFG on a pistol without it becoming an AOW under the purview of the National Firearms Act

According to the NFA a pistol is meant to be fired with one hand therefore the addition of a VFG would creat an AOW that would fall under the NFA. However the ATF has issued the opinion that so long as the overall length of the "pistol" is 26 inches or longer then it falls under the category of "firearm" rather than being a pistol and adding a VFG to said firearm would not constitute the manufacture of an AOW under the National Firearms Act.