I use either Tamiya Fine or Mr Surfacer 1200,both in a rattle can.
I have heard that Vallejo isn't a primer in the true sense,in that it's not sandable,it's more for just laying down an appropiate colored undercoat.
Correct if I'm wrong on that,but a primer that shows flaws and improves paint adhesion would be one of the enamels or lacquers.
I am using Badger's Stynylrez primer lately, it's labeled as "water based acrylic polyurethane." Using it straight from the bottle it has remarkable grip on the plastic surface, no tendency to lift when sanding.
It sands very well, either dry or wet. Also a handy filler for minor flaws, I apply it with a toothpick to scratches or pinholes. I let it dry overnight before sanding, but I have read it can be done sooner.
Easy cleanup, but it does require a more aggressive agent than acrylic paint does, since it does seem to set up more than paint. For airbrush cleanup I use lacquer thinner or acetone, in the cup and needle.
It works well for me.