You have to have a lot of baggage before the conductor can legitimately charge you extra. Never leave your baggage inside when you go topside. Develop that zen-like attitude towards pain and discomfort as quickly as possible. Traditional gender constraints tend to disappear once the wheels began to roll. I have yet to see a Vietnamese couple kissing or even walking hand in hand, but within the confines of public transport moral restraints give way to practical necessity.
People nestle like spoons and sleepy heads occasionally drop onto unknown shoulders.
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.