67 Bucket seat chrome trim

If the laquer thinner gets the all the paint off the vinyl will be clean enough. SEMs dye is laquer based anyway. They do make a prep solution to promote adhesion. It's available at the paint stores that carry their dyes. I use it on parts like kick panels that still have a lot of oil in the plastic. For very old, dried out, and brittle parts, it's not needed.
If I was simply redying the seat backs, changing the color, etc.. I would use the prep. In a case where laquer thinner scrubbing happened, I assume the surfeace oil is gone also.
I'll keep the inside of my car dry so rust wont be an issue ( fingers crossed ) so I just wire brush the insides of the set parts and coat with a rust reformer.
My way aint the best way I'm sure. Just sharing. Good luck