I also had to remove a load of vinyl lettering off my van, firstl used the heatgun and most peeled off, the residue i then removed with a rag dipped in petrol, all came off a treat.
I would certainly not use celluose thinners espcially if your van actually has any paint, as this is what car body repair specialists use to remove paint, it will strip the paint away in a second.
Petrol will take it off, and then use a little bit of brasso to remove the outline of the lettering that was there so its not so obvious it was there, or a good coat of t-cut, just remember not to put polish on until you have put the new vinyl on or it wont stick, it will also need a good few days to complety bond to the metal
