Saw some magazine grips on Larue Tactical. They attach around the magazine but my question is will it constitute a vertical foregrip on an AR pistol thereby making it an AOW?

I can't answer your question, but every report I've ever seen of an AR going kaboom stated that the magazine was blown apart.  It doesn't strike me as the best thing to hang on to.

I'll take that advise.....Like having fingers, they are useful.

http://www.ar15.com/forums/t_3_122/525036_Request___Scans_of_any_letters_from_the_BATFE__that_pertain_to_AR_pistols.html&page=2

About a third of the way down the page is a letter on the subject.  Short answer...no, it is not illegal.

I have never seen a magazine fail on an AR in an explosive manner.  Seen a few chambers blow out.  (Now THAT is exciting.)  But they were primarily due to people reloading and not paying attention to safety.

When case blows out, it seems that's one of the ways the gases get out...

http://www.ar15.com/forums/t_6_5/364365_Kaboom_today_at_the_range___pics_.html

http://www.ar15.com/forums/t_6_11/368141_1st_Kaboom____need_help_w__cause.html

http://www.ar15.com/forums/t_6_42/364739_My_first___well_second_KABOOM_Pics_in_OP_now__.html

http://www.ar15.com/forums/t_6_22/358738_Bushmaster_FA_kaboom.html

ETA:There are many more threads on here, and the results are almost always the same.  I'm not near enough of an expert to give others advise, but I don't hand on to the mag.  I'm askeered to.