As of right now, adding a VFG to a pistol with an OAL 26" or greater changes the legal definition of what it is. With a pistol great then 26" in OAL will become a "firearm", commonly called "other", once you add a VFG. The reason for this legal definition change is the fact that the firearm in now intended to be fired using two hands AND does not meet the definitions of either a pistol or rifle (no shoulder stock).
Note, that as soon as you conceal a "firearm" it instantly becomes an unregistered AOW.
Currently, angled grips are okay to have on a pistol. IF the proposed brace rule change goes into effect, then any and all types of fore grips will add 4 points to the Worksheet 4999 if a brace installed. Even a small hand stop will add 2 points per the ATF Worksheet 4999. Any score of 4 points or more will indicate a shoulder fired design per the worksheet.