In Vietnam, as in several other Asian countries (though not Japan) and many Islamic countries, it is common for people of the same sex to hold hands or to walk arm in arm in public. It is especially common for Muslim men to do this and is an expression of brotherhood. There are no sexual connotations to this behaviour.
However, public signs of affection between men and women and overt romantic behaviour in general are frowned upon. Kissing, holding hands, embracing, or sitting on the knees of a boyfriend in public are not OK and you should avoid embarrassing locals by doing so.
In Vietnam and India, don’t be surprised if you are asked personal questions such as about your age or weight by complete strangers. It is a normal form of making conversation and is not considered invasive, as would be in Europe. But sexual subjects are taboo, as are suggestive jokes.