Gary, yes Sir. This is allowed without the Pistol then becoming an NFA firearm. http://blog.roninsgrips.com/atf-letter-stating-magpul-afg-legal-use-ar-pistols/

Gary, Since this pistol is 26.5" you can put vertical grip on it legally. The ATF has long held that adding a vertical fore-grip to a pistol makes it an AOW subject to the requirements of the National Firearms Act (NFA). ... To summarize … unless your AR pistol has an OAL greater than 26 inches then you should not add a vertical fore-grip unless you first submit a Form 1 and get it approved