Question: We have three weeks in Vietnam in February, flying in and out of Ho Chi Minh City. How should we plan the trip?
Answer: Because you're flying in and out of Vietnam’s largest city (formerly known as Saigon), and you have the luxury of three weeks, my strong recommendation is that you go up to Hanoi via the usual suspects of Hoi An and Hue, explore nearby Ha Long Bay, then head across to Laos and Cambodia (visa on arrival for both, unless things have changed since I was last there).
Vientiane, Luang Prabang and the Mekong River between them are the highlights in Laos; Siem Reap (for Angkor Wat) and Phnom Penh are the key locations in Cambodia.
And from Phnom Penh you can travel back to Ho Chi Minh City overland, with extra diversions – if you have time for them – to the tunnels of Cu Chi and the resort of Vung Tao.