The rulings on Vertical fore grips on AR or other pistols has changed in the last few weeks, within certain guidelines, it can be done.
See ATF Ruling letter at http://www.franklinarmory.com/XO-26_Letter__c_.pdf
As I understand it, it works like this, the "firearm" (they are no longer classified as a pistol) must be 26" or more OAL (not including removable flash hiders, etc), it can not have a stock or place to attach one (AR's must have a pistol buffer tube), and at no time can it be carried in a Concealed manor. (define that one will ya!)
So AR "other weapon" with standard pistol buffer tube, 11.5" barrel works out to 26.5" OAL, = ok for Vert grip. same weapon with 10.5" barrel, = AOW and $200 tax stamp.
Same would apply to AK pistols, Pistol rear Trunion and a barrel long enough to bring it over the magic 26" OAL and you are ok.
Another weapon would be the Thompson 1927 Semi Auto pistols, (A5 model I think it is) and others I'm sure.
http://www.icollector.com/Auto-Ordnance ... e_i9753234
Also the ruling on Vertical grips has been tested in SC court in the past, and the BATFE case Failed. See http://www.titleii.com/bardwell/us_v_davis2.txt
paragraphs 25- 27
NOTE: I am not a lawyer, nor do I play one on TV, information above offered for Fair Use, and informational purposes, feel free to research the subject on your own and consult a Lawyer, MD or other professional help as you see fit.