First of all you need a plan.
Arrive in Bangkok, then what is your next destination? What city and what country? It might be easier to head straight to Cambodia from Bangkok and then over to Vietnam. Crossing Cambodia other than by air to Vietname MIGHT be dangerous so I would do some serious research before you try this.
You will need Thai Visa, Cambodian Visa and Vietnam Visa's as well. Depending on what country you are from, can get visa on arrival in Cambodia and Laos.
From Bangkok airport to Cambodia border is probably around 4 to 5 hours straight drive in a car. Buses will be much slower. I am not aware of any rail service going east that far, but maybe Bob or Gerald or some other person here might know.
Buses can be very dangerous as well. There are news reports almost every week of bus crashes killing all the passengers.
My advice to you would be as follows?
Set up base to stay in Thailand.
Cancel Vietnam from your trip.
You can do a day trip to the Cambodia border from Thailand or go to Angkor Wat for 2 or 3 days.
Can get to Vientienne Laos from Thailand easy enough and can spend a couple days there.
The rest of the time use to explore the real Thailand. Isan in the Northeast, Chiang Mai, Chiang Rai, Golden Triangle in the north, and then get down south to some of Thailand's amazing beaches.
This would be a much more rewarding trip for you and your friends.
You can meet people living in Thailand and share photos, videos and blogs of your trip at http://www.visualthailand.com
Good luck and ask more questions if you need more pinpoint and detailed advice