We all the standard method of triming vinyl pieces by heating them and than cutting them with a knife. Standard caveat of course being to move the part and NOT the knife.
I use a slightly different method by using a panel scriber to prescribe the cutting lines and than using a hoby knife to carve through the vinyl.
Yup, a slower method but I found that the cleanup later on is quicker and less than using he above method.
In short the same method I use for cutting bonnets and such of car-bodies.