Is it legal to put a vertical foregrip on? Yes.
Do you need to do anything before you put the foregrip on? Yes.
Letter quoted at http://www.ar15.com/mobile/topic.htm...f=122&t=458387
So, per the most recent decision by ATFEIEIO you need to register it as an AOW and pay the $5 for a tax stamp. The timing of paying the tax stamp is important, because if you manufacture an AOW without having paid the tax ATFEIEIO will become very upset and make life unpleasant for you.
Remember, this response only addresses federal law. You need to check your state & local laws to see if you are allowed to posses an AOW and, if so, what permission slips you need to obtain in addition to the federal one.