From what I've seen at 3gun matches, the few times a Vepr 12 or heavily modified Saiga show up, they decimate the times on shotgun stages/portions.
I picked mine up for fun, hunting and possibly 3 gun (none of my handguns are modified enough to compete with open class handguns, so I've stayed away from open and keep using my 590) but it is a very fun gun and superbly accurate. The iron sights are set for about 100m with 1oz rifled slugs (Remington Slugger followed by Winchester Super X being the most accurate in mine) as the ruskies intended the gun as a "hunting rifle".
Chaos makes a rail for the Vepr 12 (
here) but I've not seen anyone using it and so far have not seen any real need for a railed grip IMHO.
Haven't spent the time to pattern the gun as much as I just spent time breaking it in (literally firing 200 shells into the air at a skeet range), so no idea on if the choke would be beneficial.
The break would definitely benefit with heavier loads or slugs, as the Vepr seems to keep with AK tradition in pulling up and right.