I don't know much about sign (or vehicle) vinyl, so I can't answer that part, but we've been using ThermoFlex Plus from Specialty since we started and have been very happy with it. We've also tried their Siser EasyWeed, and that seems to be a very good vinyl, too.
We also buy the Siser flock. Specialty Materials' (the makers of ThermoFlex Plus, not the seller) flock just seems to be a little overpriced.
Don't forget to apply your 5% T-Shirt Forums discount when you order! You can find that discount code on the left of your screen here under "View Offers"
Good luck!
