We left our hotel in Phnom Penh at around 8am to make the 177 mile journey to Ho Chi Minh City in Vietnam. We were picked up in a mini bus and taken to the main bus terminal, where the coach was waiting for us. The whole journey should take around 6.5 hours, including stops and immigration procedures.
The whole journey was fairly simple, stopping once for a restbreak before the border (which took around 3.5 hours to reach). When we reached the border we went through Cambodian exit procedures (there was no que and only took about 3 minutes, the host on the bus had filled in our departure card for us) before we hopped back on the bus which then took us to the Vietnamese border (where we made another stop for food), the host on the bus had taken our passports and gone through the visa process, so when we got to the immigration counters we walked past all the ques (which were long) and straight into the country. You need to have your bags scanned upon entry into Vietnam, which took seconds, and then we were on our way, a very quick and simple entry into the country.
You no longer have to pay for a visa for Vietnam if you are from the UK, you can get a 15 day visa on arrival if you meet the given criteria (I think this is only a trial at the moment).
The remaining bus journey took around 2 hours, the scenery was pleasant which made the journey more bearable. We were dropped right in the centre of Ho Chi Minh City, the road you are dropped on is about a 2 minute walk from the backpacker district . There were lots of taxi drivers offering their services, asking for ridiculous Sums of money for short journeys (we paid far to much for a 2 minute drive to our hotel).
We used Giant Ibis busses, the whole trip cost $20, which was great value considering we skipped all the ques into Vietnam and you also received a snack and small bottle of water. Plus the bus has free Wifi too!