Never been to Vietnam,Cambodia or Laos so looking for best routes from a good entry point in Vietnam to wend to Bangkok over a 3 week period. Not looking to rush around doing a bucket list of all three. Perhaps even consider a route that flies into north Vietnam and out of south and wending about that way.
That will be a difficult itinerary. I answered this before, and it failed to load correctly, so here is my quick suggestions. The first one would be a lot more leisurely but doesn't include Laos. The second one does but would eat up at least 3-5 whole days on buses, and there would not always be night buses connecting destinations. Just message me if you'd like more details.
HCMC: 4 nights
Chao Duc/Long Bihn: 3 Nights
Phnom Penh: 3 Nights
Siem Reap: 5-6 nights
Sisaphon/Bantaey Chmar: 2 nights (info on this here--it is a bit off the beaten path but not really out of the way) http://www.southeastasiatraveladvice.com/2010/11/banteay-chmar-travelers-tale.html
Battambang: 2 nights
Arayanaprathet: 1 Night
Hanoi: 3 Nights
Luang Prabang: 4 Nights
Vientaine: 4 nights
Pakse: 4 Nights
Kratie: 2 Night
Phnom Penh: 1 night
Siem Reap: 3 days
If you have it in your budget, you could fly from Vientiane to Phnom Pehn and spent more time there and in Siem Reap. Vientiane to Pakse is a very, very long trip, and it is basically another two days on the road traveling to get from Pakse to Siem Reap. If you are coming in the rainy season . . . like now until about mid-September or so . . . it will probably take even longer.
Hanoi is a long way from Bangkok almost 8 years ago
I'd agree with Jarrod: that's a lot of travel for 3 weeks. My recommendation would be to explore HCMC fr a few days, then explore the Delta - there are loads of nice towns and villages (Mytho, Cantho, and many others) ... I'd allow a week for that alone. From here you could cross into Cambodia and do Angkor Wat, easily another week, then possibly fly direct to Luang Prabang for a few days.
I really is very easy to spend 3 weeks exploring any one of these let along all of them together.
Slow travel gets my vote! almost 8 years ago