I haven't even worked with a vinyl kit yet, so I haven't stripped one either.
I do know over the years, I have heard lots of people say they have no trouble stripping vinyl with Castrol.
Now, as to the plaster, I have no idea.
Maybe you could try basting the kit with the castrol and see if that will lift the paint.
It would avoid worrying about it soaking into the plaster.
Castrol is fine on vinyl. I had to use it to strip the damaged areas on my Steel kit. It did a beautiful job and had no reaction to the vinyl. As for the plaster, I don't use the stuff in my kits so I don't know. I would tend to think that it'll soak into the plaster and possibly weaken it. If that happens you might have to clean it out and pour in fresh plaster.
Plaster is pretty absorbant stuff. I'm not sure if you'll be able to flush the Castrol out of there. You might want to coat it with something first, but I don't know what will stand up to the Castrol Super Clean.
Hmmm...thanks for the warning. It's that darned Troughton kit from Comet, never was happy with it. It's time to try it again.
I almost posted that at least I can stand the lower half in a jar and soak it without the stuff getting inside to the plaster...but then there are holes up into the feet to screw him to a base. If I can figure a way to plug those holes so the Castrol can't get up through them...?
Think I read once that Castrol also eats away putty. Don't know if that would include epoxy putties.
I don't know about epoxy putty. I might give it a try to see what happens. I stripped down a Starcraft's Reliant last week and it had some bondo spot putty on the nacelles--washed down the sink right along with the paint!