I want to backpack Southeast Asia?

I want to do Vietnam, Laos, Cambodia, and Thailand. Am I able to buy a 1-way ticket to Saigon and just travel around, or do I need to show onward travel when I land in any of these countries? Also, would I be able to buy a round trip ticket to Thailand for say 90 days, and then travel to Laos, cambodia, Vietnam and back to Thailand by land? (the max stay for flight into Thailand is 30 days)