This is where I got my visa for Vietnam:

Lucky! Lucky!
413 Monivon Blvd
Phnom Penh
- Visa Extension & Air Ticket
- Cambodia Visa Extension
- Vietnam, Thai, Laos Visas
Tel (023) 212 788, 2200988

The agency is right next to the Big Luck hotel. They hire out motorbikes, but also run a travel agent, apart from that, it is possible to exchange money here. Getting a visa for Vietnam is worldwide, most uncomplicated, and quickest in Cambodia. For an extra fee, one is able to make use of the express service; the visa is issued on the same day. I wasn’t in that much of a hurry, so I was issued with mine the next day. One has to hand in ones passport. Later on, I noticed that the visa had been issued in Sihanouk Ville, they probably have a courier. The cost of a visa is quite low, a lot cheaper than getting it done at home. Here I also purchased my flight ticket to Saigon (Ho Chi Minh City), the cost of the round trip, Siem Reap – Saigon – Phnom Penh, including the visa, was 250 US dollars. 

By the way, it’s quite cheap and easy to catch the bus to Saigon from Phnom Penh, costs between five and nine US dollars. The journey takes around about five hours. Quite an interesting offer, during the journey, one gets to see quite a lot of the countryside, of course a lot more than when flying. I wanted to go to Siem Reap first, so I bought myself a bus ticket down at this travel agent for nine US dollars. Because all of those making the trip were stopping in various hotels, I was picked up first by a mini bus. This service was included in the price. If one purchases the ticket directly from the bus company, it only costs six US dollars.