depends on what you want to do / see, if you've never been to Angkor What, I would go from Bkk to Siem Reap , then take the boat trip to Phnom Penh , then bus or fly to Saigon. Can be fun to go to sianoukhville from pattaya, then stop in phnom penh and down to saigon. Last resort would be direct flight from thailand
