I just remembered to post this tip after replacing some wind0w vinyl today , 20 30 g maybe not as big a deal but I see some heavy vinyl 40 60g and this old vinyl was hard and brittle.
after I sew on new vinyl retaing the same shape I cut the old out and will use my sissors sliding around the edge but being to hard it was imnposible so I use my engle hotknife with no foot and carefully slide the hot blade heating the edge and then my sissors can easily slide slicing the vinyl.