How do I paint over a vinyl canvas that already has been painted?

Here's an idea:

https://thriftdiving.com/diy-wall-art-from-thrift-store/

You absolutely can! Would love to see what you do!

I would spray paint then white flat or paint them flat white

Artists use Gesso. You can get it at art supply stores or Michaels, Walmart, Amazon

Yes, I've painted over them before! Any white primer should work as a base.

Yes use any household primer. I prime thrift store paintings all the time. The price for frames and canvases are fabulous there.