One of my friends is Pat Joynt from Saigon Scooter Centre. He says that all the good ones, particularly with Lambrettas, are gone now. What's left is the rubbish. As much as Pat tries to guarantee his work, even he has been caught out. You see, a business may be run by a European/Australian/American but the work is done by a Viet and whilst some of them are good, some will cut corners. I know of one machine in Sydney that Pat supplied that he ended up sacking staff over.
From Vietnam, I really only know of 2 companies really run by foreigners. SSC and ScootRS. The rest are just front pages, such as lambrettas.com.au - heard VERY bad reports about their stuff.
Essentially, the only way to get a guaranteed good old scoot is to:
1) Buy one from someone you trust and know is a good mechanic
2) Buy one from someone who bought one from a trusted and good mechanic
3) Buy a barn find, and give it to a trusted and good mechanic
4) Do it yourself, if you trust yourself to be a good mechanic
Remember, you are often dealing with 40 to 50 year old machines. To expect them to be as reliable as a modern P series, unless you've had a complete rebuild, is naive at best. It took Piaggio 30 years to get the P series as reliable as it is today, and that's on top of the 30 years of previous experience they already had.