OK, I think I found what I was remembering, MooglyGuy logged an issue for vector core improvements, and the thread seems to explain a good deal of what I was thinking about why MAME can't yet reproduce some of these things that are part of the look of vector monitors. https://github.com/mamedev/mame/issues/399
That said, even with this I'm still not fully clear on what obstacles remain, or what work still needs to be done. If he or anyone else has any more info, though, please feel free to share it here. Thanks!