Should i lay plastic between the vinyl and foam when reupholstering my boat seat?

Better to use top quality material(s) and good dressing compounds - I haven't found extra layers any benefit with the quality covering materials, although the cheap stuff probably needs it. The use of an open weave underlay beneath any cushion for continuous ventilation will help a lot. The sealed surface closed cell foam is always worth it for any exposed upholstery as it is not a sponge. Again if you are going to all the effort you may as well do it right I think.

It provides a vapor barrier between vinyl and the foam. The foam is,of course,porous and so is the vinyl to a degree. The plastic is not. Not allowing moisture, water, condensation etc. to penetrate deep into the foam will allow the seats to dry faster and reduce the time the moisture has to start the rotting process.