On entering Cambodia a US citizen can purchase a one month tourist visa, make sure you take along a passport photo.
For Vietnam, you will need to apply prior to entry. You cannot obtain a visa on arrival and will be deported if you try to enter without one. Getting a visa can either be done at the Vietnamese Embassy in the States, or if you plan on being in Cambodia first, you can arrange one through the embassy there. It will take up to 7 days (at most). For additional fee you can have it processed within 1 day. Your hotel may be able to assist with facilitating this as well.
You also must have a passport with at least 6 months left on it.
As for Costa Ricans - not sure.
You may be able to find some more info here:
http://cambodia.visahq.com/ about 9 years ago
I agree with Lisa above, but I would get the visa before you go.....just to be sure, you want to keep your passport with you while overseas. I know in Canada the 7 day wait also means you have to send your passport away to obtain the visa, or leave it at t he embassy. ( in north America, you know it's safe to do so) about 9 years ago