I too was searching for a good primer for vinyl. I called Krylon and Plasti-Kote but they said that their primers would not go good with vinyl, or acrylics for the rest of the paint-job. After visiting a automotive paint store, I decided that I would try a Flexible Primer Surfacer #39133 made by SEM Products. I called the company and they weren't really sure it would work for vinyl kits. Well, the man at the store who sold it to me said that he knew it was sometimes used for car vinyl tops, and of course bumpers and other flexible surfaces that the stuff was intended for.
I was a little hesitant to spray but I think I remember the guy at the paint store saying it was a lacquer based primer. Being that fine folks in this forum have stated that lacquers actually do go well over vinyl, I figured it was safe. I sprayed a part of my kit. I accidently oversprayed one part but to luckily no meltage (lacquers being hot which could damage some plastics if applied heavily). It dried quick and firm, something I would expect from priming normal styrene models which I am more used to building.
Although Krylon said not to try their paints on my kit I went ahead and sprayed a spare piece and although it didn't stay tacky, it didn't feel very well cured and I wasn't happy with the feel of it.
I hope this little bit of info helps somehow...I know exactly how it feels when building a vinyl kit. But I can't complain, it really is a lot of fun. Peace!