I've played Pokemon Go in Cambodia so I can give you some insight.
Farfetch'd is not available in Cambodia. Only in Japan and South Korea.
Corsola is possible but I never found one while I was there. However, if you happen to fly to Thailand (as most planes that go to Cambodia usually stop there first), you can find Corsola (that's where I got my first one!)
No idea about Vietnam, but my guess is that all countries in that area (Thailand, Laos, Cambodia, Vietnam, etc.) have the same type of spawns.
Have a wonderful trip to Cambodia! If you're in Siem Reap or Phnom Penh, you'll find excellent spawns! Just be cautious while in Cambodia though; if you are caught playing at holy sites or at The Killing Fields/S-21 Museum in Phnom Penh, you could be fined or jailed.