There are some flights connecting the main cities of Vietnam with Siemreap and Phnompenh. Travelers can fly with Vietnam airlines or Cambodia angkor air.
If your time is not so limited, you can take the bus from Ho chi minh city to Phnompenh or Siemreap. It takes around 6 hours from Ho chi minh to Phnompenh and 14 hours from Ho chi minh to Siemreap. Or from Ho chi minh city, you can travel by land to Chau doc  (245km from Saigon), then take the speed boat from Chau doc to Phnompenh. Ticket costs around 30 USD. The boat trip takes around 5 hours.You will do the immigration on the Mekong river along the way. Cambodia visa is available at this border. You just need to prepare your original passport, 2 passport-size photos and 30 USD by cash. Then the boat staff will help you to get Cambodia visa.