I have bought over a dozen scooters from Vietnam.  Each and every one of them inspected by a certified Vespa mechanic here in the U.S.  Of course you need to be careful when purchasing anything sight unseen, but I have had just as many bad experiences buying here in the U.S. as elsewhere.  There are liars and cheats everywhere, not just in Asia.  It pays to do your research and buy from someone who has experience in the field.  In short if possible look at and test drive the scooter, check reviews from past buyers, and if you are still uncomfortable look in the phone book, find a mechanic near where the vespa is being sold and pay to have him/her drive out and take a look.  Thanks for your time and don't let one person's bad experience bias you against half the scooter on earth.

Thanks and Good Luck.