i used vanish on mine once, although i think the general consensus is not to do it as the fibres in the belt can be destroyed or frayed = risk in an accident. but then when you think about it, in some accidents people survive who dont wear them! lmao.
when i used upholstery cleaner, i just swept down ways not side to side, because if you look at the belt, the fibres seem to go mainly downwards (well isnt that logical? lol)
just be careful, they are seatbelts afterall.... (i never said do it! lol)