I haven't used a VFG on a shotgun, so take my comments with a grain of salt; but one of the problems I ran into with ARs is that if you have your hand only on the VFG, that is a lot of weight rolling around on top of your fist and it makes recoil control (sights go straight up and straight down under recoil) difficult.
That arrangement puts 4-5 12ga shells in a tube above your fist, the bore even higher over the VFG, and considerably more recoil. So my first instinct would be that it works poorly for accurate, quick follow-up shots; but I could be wrong.