What time of year? If thinking of Vietnam, Saigon is always hot (with dry and rainy seasons) whereas Hanoi is much more comfortable outside of the summer months. Similar considerations apply to other countries in the region. If you're sticking to posh hotels or shopping malls in Singapore, then heat won't be an issue. If you're outside it could wear you down if every destination is hot.

I suppose for 2 weeks, I'd go for 2 or 3 cities/locations. I hate bashing around trying to fit in too much.

Otherwise it depends what you're into - beaches, history, culture, walking through landscapes etc. Also budget - a nice hotel in Vietnam/Cambodia will cost less than a nice one in Singapore...