Before you enter Vietnam, you will need to have the visa already in your passport – you cannot get a visa on arrival, there is no such facility. Here’s a post about how to get one in Bangkok Get A Visa For Vietnam Before Crossing The Border, you’ll need at least four days for processing.
Any travel agency from Cambodia will be able to arrange a bus for you. Once you board the bus, the conductor will collect your passport. Don’t worry, its fine and safe to hand it over – he needs it for the Cambodian side of the border.
Approximately 3.5 hours into the journey, the bus will stop not far from the border for a breakfast break, you can eat, drink and use the bathroom there.
It’s weird, but just play along.
Two minutes drive from there, you will arrive at the Cambodian side of the border. As the driver has already collected you, don’t need to do anything, nor even pass through immigration. The bus will stop at the entrance to the border, you will get off the bus. The bus will then drive to the exit of the border, you will get back on. It’s weird, but just play along.
Once everyone is back on the bus, the driver will take you to the Vietnamese border and all passengers and their luggage will be unloaded. Don’t worry, the baggage handlers are trustworthy and will put the bag on the correct bus on the otherside.
At this point, the bus conductor still has possession of your passport. Go through the immigration office, which looks like this.
Your passports are all processed together by a single immigration officer, he checks the visas and stamps your passport. They will call your name, one by one. Collect your passport and walk through.
If you have any bags at this point, you will need to put them through the security scanner. The attendant is probably on his phone, but be careful with what you carry!
At the door, a final officer will ensure you have the visa again and verify the stamp is dated correctly. You may then go through to Vietnam.
Remember the number plate of your bus and wait outside until you have seen that your bags have been loaded onto the undercarriage.
The bus will take another 2 hours from the border to reach Saigon.
Congratulations! You now know how to travel overland to Saigon.