Frost Auto restorers do a range of interior restoring products (at <A target="_blank" HREF=http://www.frost.co.uk/productList.asp?catID=21>http://www.frost.co.uk/productList.asp?catID=21</A>) which includes a paint that can be used on vinyl - good stuff, though not particularly cheap.
There's a vinyl paint you can buy in halfords called foliatec. Brilliant - have changed the lovely american pink in my boat to navy blue - it seems to sink into the vinyl so you can still see the grain - you can't tell it's been painted.
I know somone who used it on a motorcycle seat and it never wore off, that's an environment that's pretty much as hostile as a boat.