I started looking around for a certain type of receiver block a few weeks ago to use with a VIS, and happened to notice ADCO had one for a good bit cheaper than the others I was looking at.  ADCO says they won't sell junk that doesn't work, so I ordered one.

With a block of wood you can rebarrel flat top uppers and VLTOR VIS's, with the upper sideways in the vise (good for the VIS, since access to the barrel nut is from the bottom)  Just used it; it works perfectly.    If you don't plan on doing A2 uppers, it's all you need.