Visa is required for citizens of Hong Kong to visit Vietnam. Hong Kongn nationals may have 2 options: Visit Vietnam embassy in Hong Kong or apply for a visa on arrival to Vietnam from Hong Kong online. Hong Kongn citizens may apply for both tourist and business visa to Vietnam for the stay of 1 to 6 months from Hong Kong. Please check this link for visa fee for Hong Kongn citizens.

All the information for getting a visa from Vietnam embassy in Hong Kong can be found here: http://www.vietnamconsulate-hongkong.org/