Yeah try the warm soapy water but do a test on an inconspicuous area first to test how rough you can be with the rubbing. The problem with vinyl is that it is a rubber like substance bonded to a cloth, it's extremely reactive to heat so make sure the water isn't too hot as older vinyls break down really easily if treated too rough. Try to use an old stocking or natural fibre rag as a cloth DO NOT use a dyed fabric as you run the risk of transferring the colour of the cloth you are using onto the vinyl, at which point you're stuffed.
Once again I wouldn't touch it with any chemical based cleaners but if you do try it on the back or in a spot you won't notice if it damages it. It's hard when dealing with old stuff, sometimes it best left alone.
They look cool none the less, rockin retro artwork.