From a technical approach, there are three major things that break down (loss of plasticizers) vinyl- heat, chemical attack, and UV. Unfortunately there is nothing you can use that will reinject these plasticizers back into your vinyl- Once they're lost, that's it. One might call it natural aging which is inevitable. The best way to preserve your vinyl is to keep it cool and dark with no treatments. I'm no expert, but my experience with polymers is that just about everything attacks and breaks them down in some shape or form starting right after they're manufactured. Some polymers are just better at resisting specific attacks than others. With that said, it's impractical to think you can seal up your dash in a tomb. The question is- are you willing to risk chemical attack to make it look nice temporarily? The way these products are sold is that they protect against UV, which is true, but most, if not all are a double edge sword that end up breaking down your vinyl. If there are any chemical engineers here, I'd love to hear your take.
Shine 'em up boys, but don't overdue it.... and cover them in the sun if you can. My .02
