It's pretty easy and hassle-free to travel between Phnom Penh and Ho Chi Minh(HCM) city, vice versa on board a bus. If your journey is from Phnom Penh to HCM city, you can get the bus ticket from the main bus terminal located adjacent to the Central Market, right at the heart of the city. Ticket costs USD12 for 1 way trip. The Phnom Penh Sorya Transport Company's bus service is reliable, clean and the bus condition is satisfactory, which also comes with a free small bottle of mineral water. The service also comes along with a guide on board the bus. The guide will assists in getting the stamp on your passport done without you having going through the hassle of lining-up in case of overcrowded situation at the immigration exit/entry. It was quite a pleasant experience for me when I entered HCM city from Phnom Penh at the Moc Bai(Cambodia)/Bavet(Vietnam) border checkpoint. However, situation was quite chaotic from the opposite direction. (back from HCM city to Ohnom Penh), where people were seen jumping ques and there were some pushings here and there couple with minor confrontations among the travellers. Apart from that, it's really a convenient and cheap way to travel between the two countries. In HCM city, you can get the bus ticket from the same bus company at their office located at Pham Ngu Lao St. or you can opt for the bus service from the Sinh Cafe at De Tham St, which I believe works with the same bus company to provide transport services between HCM city and Phnom Penh. Costs for return ticket is also at USD12. Arriving from Phnom Penh, you'll be dropped off at the backpackers area in Pham Ngu Lao St. On return to Phnom Penh, you will have to wait the Sinh Cafe office if you purchase a ticket from them and at the Phnom Penh Sorya Transport Co. office if you do with them. Hope this help.