1

From what I remember most bikes had a lock, one that goes through the rear wheel, you push it through the wheel and then remove the key or they should give you a chain with padlock. Never had a problem with bikes in SEA. If you want you can bring a cable that will go through both wheels and then attach the bike to a post or something.

2

In Vietnam use a bolt cutter proof chain and a bolt cutter proof lock to chain the bike to an immovable object. (Even if you lock it alibaba's relations will carry it away).alibaba has a lot of relatives in Vietnam.

3

Forget the USA, especially as far as stealing bikes are concerned (I had 2 stolen in New York, where I lived for 35 years) !!!!!! No bicycle racks anywhere (maybe in Saigon, but not in Hanoi anyway !). Rental comes with a lock (if they don't, ask for one or rent somewhere else), to be used during the day. At night, the hotels put the bikes in the lobby or the storage room if they have one on the street floor (even motorbikes in some of them !!!)

In many touristic places outside of large cities (for example in Tam Coc), most hotels have free bikes

I agree with Herbie ; I live in Hanoi and use a bike every day, and I never had a problem in 5 years !

4

At night, the hotels put the bikes in the lobby or the storage room
Otherwise thieves will steal them!
Most Vietnamese put their motorbikes (and cars) in their living rooms at night for the same reason.