The only vanities that are worth a lot are the ones that were discontinued before they made them tradable. Back then people had no reason to buy more than one of each vanity, so nobody had extras and thus they are now worth a lot because not many are on the market.

The vanities that came out later on seem to go up in price a bit after they are discontinued, but the only way they really keep increasing is as long as players quit the game and new players enter the game. So the price increase is very gradual. Your founders sets fit this category. I would expect they will continue to increase in price at a very slow rate. So no reason to hold onto them if you need the cash for something else more important.