u will need to get a visa for both countries , cambodia u can get on arrival at the airport and some border crossing , you best best is to have a look at the visa section on this web site
To get from Siem Reap to Vietnam (specifically Ho Chi Min) you'll have to go through Phnom Penh , so it might be worth factoring in a day or two in the capital to break up the journey. As the previous response noted, visas to Cambodia can be obtained on arrival (less hassle in the airport than land border), but most nationalities need to obtain Vietnam visas in advance, as they're not available on the border. You can take buses from Siem Reap to Phnom Penh and then from there on to Vietnam. If you want to do it all in one go and just swap on to another bus in Phnom Penh, you'll be able to buy a ticket for the whole journey at any guest house or travel agent in Siem Reap.
#3 sarahincambodia has been a member since 17/9/2012. Posts: 1