I stayed at the Mad Monkey Hostel in Phnom Penh and booked my ticket over there. There where 3 options to of busses. Local bus ($13), an touringcar($18) and a minivan($15). I took the touringcar with wifi and comfortable seats. It was leaving at 7 o’clock in the morning. (little bit hard after a good beerpong play in the Mad Monkey bar)
In the bus from Phnom Penh to Ho Chi Minh
In the bus from Phnom Penh to Ho Chi Minh I wrote a blogpost and managed my photo’s so I didn’t wast time. But it cost $3 more. At the border from Phnom Penh to Ho Chi Minh the bus driver took our passport and checked the Vietnamamese Visa. He did the work for us and we could stay in the bus just waiting. After that we could walk with our bags to the border and get in front of the line. It took 5 min wals and we were back in the bus. The ride took 7 hours in total including the bordercross.
Or book your busticket from Phnom Penh to HCMC yourself easily.