sorry for the late reply i'm toying with the idea of throwing out my bis lump and going down the hot-rod route after it burned it's head! it's breaking my heart at the minute but back to the vespas, there is a little washer beneath your clutch nut which should have an edge of it bent up to stop the nut spinning loose if you tap this down flat the nut should come off handy enough,if you've tried this and it's not working are you using a flywheel holding tool to stop your crank turning??
hope this helps if not get back to me and we might sort some thing out!