The best way to not get taken for another ride on a rental bike is to use someone that has been around for a while. Passports for security deposit are the norm and "fake" passports are not generally an issue as most agents will insure that is the actual passport you used to enter the country.
A few tips on rental:
- Inspect the bike for damage prior to riding off. Note any large gashes in the plastics or out of the ordinary wear and tear issues
- Look for an insurance sticker. If there is not one, ask the agent.
- When staying overnight somewhere, inquire where to park the bike. Lock it up if you can and be confident on the parking. Is it next to a security guard? Is there a locked gate? Is it inside the reception area of the hotel/guesthouse?
- Be clear on your pick up and return dates/locations. Most rental companies will have you pay in full for the bike before you leave and early returns are not likely to be refunded. This is because the bike is out on rental and not available for others and once out of the rotation, some companies only have a few bikes and idle time isn't in their best interest.
- Inspection ride the bike. Go 2 or 3 km on the bike. It's usually not a problem and you'll get a feel for if the bike is damaged somehow making it unsafe or unfit to ride.
- Keep copies of your rental agreement and passport with you at all times. Read your rental contract closely. Repairs to bikes on the road during your rental time are sometimes not covered if you don't inform the agent there is a problem.
- Get confirmations before you arrive. Make set meeting times to get your ride and look over the forums online for reference to who is who. A small shop in the middle of Ho Chi Minh City might not be as expensive as one of the more used places and often it is a matter of reliability vs. price.
- Use your head while on tour. Riding areas that are way above your ability or posted OFF LIMITS will just lead to problems and possibly the bike being held by police or military. The owner of the bike will need to get involved and this isn't easy at distance.
In general, the forums you can look over have names and references of reputable companies.
Ride safe, ride smart.