Never used a Vulcan receiver before, but have heard about people having to "egg" out the holes on some of them. Not sure if it was the rear trunnion holes though. I think HotBarrel has built with a few Vulcans, but I could be mistaken.
I have allways had to egg out at least one or two holes with vulcans. how far off are they? it is ok to egg out a littel but if they are way off then you are looking at fill with weld and redrilling. If the holes are totaly off you can just drill new holes and leave the old ones if you do not mind looking at empty holes. I left one empty hole in the reciever on my AMD65 build because of the differant rear trunion. I think it looks fine, and I guess I can fill it later if I cant stand it someday.
Think of a Vulcan as a rough start NOT a ready reciever!!! not too bad for the price.