You can only ever go in one direction: SAVE to MODSAVE. You cannot take any modded characters back to vanilla. If you just want the one character and not the shared stash, just find that character's .SVB file under the SAVE folder and copy it over to MODSAVE.

Couldn't you theoretically use the Mod Unbinder and remove any items that were from the mod via console commands, and then put the .SVB file into the save folder and keep playing as though it was unmodded again?