As a foreigner, you will be largely exempt from this. If someone's really crowding you, expect a thief. Never put baggage under your seat. A thief can get at it from the seat behind you and slip off the bus. Never put baggage in the overhead rack unless you have a means to secure it.
Thieves are fast and efficient.
The Vietnamese are respectful and physically aloof. The man was as quick and agile as a cricket, leaping down and jogging alongside, scooping up babies and manhandling women up the steps. When we pulled over to pick up larger groups with bicycles and cargo he clambered up on the roof like a gecko. A helper slung the bikes up and he snatched them out of the air and lashed them on board in record time. I pulled out my watch and clocked his best efforts.