Have been using B&Q "Kitchen" emulsion on the vinyl headlinings with some success. leaves a slightly tacky surface which improves after a second coat.
Would have reservations about using it on cushions though.
Vinylkote 40 is the stuff your after. Not cheap and will soon wear off, especially if you wear abrasive fabrics like denim etc. You can pick it up in most car shops or coach trimmers.
Follow the instructions on the tin. You'll need to cleen the surface you want to paint. The thinners for it is ideal, pongs a bit though. Then apply the "paint" in a couple of thin coats.
Its not that brilliant on seat cushions as they flex when you sit on them. You'll need to do it every year or so depending on how much they get used.
Your probably far better off having your cushions recovered for a lasting solution.