> I wonder if Asimov considered multivac to be an ancestor to his positronic robots, or if the two exist in different universes. I don't recall the two ever appearing in the same story.

I can't remember if the machines in "the evitable conflict" are ever called VACs, they might be. The themes in that story do for sure overlap with the story "Franchise" (which is explicitly multivac).

Anyway the multivac from last question probably isn't the same as the one in franchise anyway, because the franchise multivac is the same one as in "all the troubles of the world", and spoilers, but that particular multivac has other problems than entropy. It could be that they "fixed" it, but at this point the timeline with other short stories doesn't add up.

In any case, the VACs would be instances of positronic brains the way the machines in evitable conflict are, so if anything the robots are the ancestors of multivac and not the other way around.