Use the heat gun VERY GENTLY!
If you overheat the stickers, the vinyl will come off easily, but leave all the glue on the vehicle. Just warmed slightly, most of the glue stays on the vinyl, so you have a lot less to clean off the vehicle.
Just putting the vehicle in the sun on a warm day for a few hours will soften the glue......... but a heat gun is faster. Remember, though, warm, not hot!