Here's the deal. Tomos put the vin on two foil stickers, one on the head-tube and one on the frame area a few inches below the seat. The bad news is that there were frames that had no stamped vin number, just the two foil stickers. Those stickers fade and deteriorate if the bike was left outside for a long period. On some models the vin is stamped into the right top motor mount which is behind the air filter on bikes with the offset intake. It's not the full vin, but it's the unique number to that specific bike and should match the number on the motor. If you have that number, the color of the bike and can figure out the year you can find out the rest of the number.