Visa is NOT required for citizens of Cambodia to visit Vietnam in less than 15 days. for stay of longer than 15 days, Cambodian nationals may have 2 options: Visit Vietnam embassy in Cambodia or apply for a visa on arrival to Vietnam from Cambodia online. Cambodian citizens may apply for both tourist and business visa to Vietnam for the stay of 1 to 6 months from Cambodia. Please check this link for visa fee for Cambodian citizens.

All the information for getting a visa from Vietnam embassy in Cambodia can be found here: http://www.vietnamembassy-cambodia.org/