I was solo backpacking in Cambodia for 3,5 weeks and in Laos for 3 weeks this year. Both countries are fine to travel alone. read more
Visa requirements are a bit more strict (you have to apply in advance) and don't offer VOA like Cambodia and Laos. Besides, for Vietnam three weeks would be better as the country is so big. Both Laos and Cambodia have different things to offer. The big draw to Cambodia is surely Angkor Wat. read more