It may not work on your particular vinyl adhesive, but I always found WD-40 to be an excellent remover of adhesives. It worked well on removing old duct tape/racers tape that I had forgotten to remove before leaving the car resting inside the trailer during the different seasons.
Heat gun the best to use, after first attempting to peel it off. As someone said earlier, lay the decal almost flat over itself before pulling it off, pulling almost horizontally to the surface. Heating as you pull it off. Pulling it vertically, straight up, only results in a lot of little vinyl pieces and paint pulled off the car.