I removed a vinyl heat press shirt number from my son's football shirt by turning it outside in, placed a clean dry dish towel inside to stop anything soaking though to the other side of the shirt, lay on a flat surface and soaked the area with Acetone (not nail polish remover) pure undiluted Acetone, any hardware store, chemist, supermarket etc sells it, as it is regarded as a general household solvent, and costs about 1 or 2 pounds?, I then pulled the soaked number area stretching back and forwards to loosen vinyl, after about 5 mins the vinyl number peeled off no problem, however I was left with a glue residue in the shape of the number??, well I turned shirt inside out again, placed in a plastic bag and put in the freezer for 24hrs, the cold freezes the glue making it brittle, after 24hrs I took the shirt and lay it on a flat surface and with a blunt butter knife (or something similar) and scrapped off the glue residue while it was still frozen, only took a couple of attempts but worked a treat if your patient and persevere.
Sam