The in-game despawn is on a thirty day timer, so if you want cells to reset, you need to wait thirty days in game. You need to be in a cell away from the vine man, however, as local cells seem not to reset with you in them.
Otherwise, I'd try forcing the body off using your Unrelenting Force shout (Fus) or the spell Telekinesis, if possible.
Worse comes to worst, load a previous save and try again.