Steven,
I think this might be fairly difficult since the "Car Registry" does not include the former owners Forum / Site UserName. The registry, however, does show the privious owners First and Last name. Only you would know what that is since you mentioned that you found the VIN number of the car you now owne in our registry.
I suggest you post the question here as to if anyone remembers the First and Last name. Chances are one of the long term members might remember the former member and his User ID.
If the previous owner is still a member here is another question. If he/she was no longer active for a number of years then his/her profile and file would be deleted since from time to time we clean up the data base.
Good Luck