The World Co-ordinator in "the evitable conflict" was a positronic robot (not known to the public), but I think you're right that the machines are never identified as either positronic robots or VACs. But iirc, in the Susan Calvin universe (of which "the evitable conflict" is a part), robots were generally illegal on Earth, the that must make the machines in that story non-robots.

I would say the multivac in "Franchise" is the same Mutlivac as "Last Question" and "all the troubles of the world" (one of my favorites). There are no positronic robots in "Franchise", nor the others.