AFAIK, the Blue Dots are supposed to be closer to the current run of 2520s than the Red Dots which are supposed to be closer to the vintage ones. I seem to recall reading that in one of the many threads. So, I believe the thinking would be as follows:
VP312 w/ Blue Dot = closer to the 512/3124
VP312 w/ Red Dot (or GAR, etc) = closer to the 312
I'm not positive, but maybe Jeff can chime in to clarify?
As far as the build goes, I built a pair of VP26 first but I don't think they're a prerequisite to the VP312. In fact, I found the VP312 to be slightly easier than the 26. I posted a thread about the differences (w/ photos) a while back that may be helpful to you: My Classic API VP312 Build