Don't fill up too much with plaster. If you drop it, it might shatter, vinyl or no vinyl. I've done many of these types of kits and never had that happen, but I've heard stories. (The kits I did were all first runs, not potential knockoffs, but I am not casting any judgement. just FYI)
You can check out my assortment, none of which I actually own, all were done for clients, at
RGP FX Gallery - Models, Miniatures, Costumes | Facebook
All the other advice sounds good. I tried the great-stuff expanding foam filler once on a hulk, but there must have been a void in the head area and it actually shrank!. I was able to slice an airhole in the back of his head and the blow drier returned him to normal. Made him calmer too.
For the rest, I stuffed with paper towels and glued with little tubes of liquid type super glue.
I did not notice anyone touch on it, but vinyl is sensitive to certain solvents, especially acetone and the solvents in most aerosol paints. I usually used a water based primer by DAP that is no longer available, or later I usually did a few light dust coats with Testor primer and then gave it an airbrushed acrylic base coat. With a coat of acrylic, the vinyl is safe from lightly applied enamels, like metallics, that need to go over certain areas.
Good Luck,
RGP