In the Vietnamese culture, whistling is done when a person feels happy, when he wants to attract other people's attention in an informal way. Sometimes a person may whistle along with a song so that he/she can feel relaxed.
Hand-clapping is also used in different situations. The first one is that when we want to encourage someone, we clap our hands. And the other is when we want to despise someone. However, the difference in the way we clap - fast or slow - together with other expressions like facial expression or sound will make it easier to imply the meaning of the hand-clapping.