There's an immigration office that manages visa applications. For this reason, you should pick an appropriate visa type for your goal. As a way to be legally employed, you must have a a Cambodia work permit, but you don't require a work permit to receive a visa.
Also, take note there are restrictions on when it is possible to re-enter Vietnam using a visa exemption. US passport holders must get a valid Vietnamese Visa in order to join the country. Work visas don't need to be overwhelming though!