Also any advice for 3weeks what to see and do in those country for that time. I will be flying from namibia/ south africa
Usually the best time for countries in that region is between November and Feb/March as it's the dry season.
I haven't been to Vietnam but in Cambodia I'd suggest biking around Angkor Wat in Siem Reap, or heading to Otres beach in Sihanoukville.
I'd also highly recommend Laos if you can find the time! about 3 years ago
In my opinion, 3 weeks isn't long enough to do both countries. I went to Vietnam in September for 2 weeks (weather was great then btw) and it wasn't long enough.
In terms of where you should visit, Ho Chi Minh/Saigon is a really cool city steeped in history and a day in the Chu Chi Tunnels is a must as are the war museums if that's your thing. Hoi An is really beautiful and you can do some bicycle tours out into the countryside which is really cool. Sapa is also a must see with some great sights and photo opportunities.
The weather in the north and south of Vietnam is very different so whilst the south might be really warm, the north might be cold, so try to go there at a time when its good weather in both (Feb-Apr or Aug-Oct).
Vietnam is a really beautiful country so I hope you take your time exploring and enjoy! :) about 3 years ago
February is probably the best time to visit Cambodia. Not to miss: Siem Reap (3 days is ideal), the boat to Battambang from Siem Reap, The Killing fields in Phnon Phen, Sihanoukville, with a boat trip out to the islands, and Kep. Definitely don't have time to do both countries, both are beautiful but you need at least 3 weeks (if not longer) to see Vietnam. about 3 years ago