If you intend to cut straight lines in the vinyl, it's far better to use a straightedge made from a thin strip of wood veneer. A metal ruler or the like draws too much of the heat out of the iron tip and will consequently make the process extremely slow.
I was able to find some thin, 3-ply veneer at a craft and hobby supply. Works just great.
+1: make sure the iron tip is dull and very smooth.
Pull the protective vinyl off of everything and throw it away and never look back.
Pull the protective vinyl off of everything and throw it away and never look back.
I agree with Vince!