The Vietnamese Consulate in Guangzhou handles visas for Hong Kong people.

The building 1 will kill people. Hong Kong passport to Vietnam is not visa-free. You can apply for a visa at the Vietnamese Consulate in Guangzhou. Passport, two one-inch photos, a copy of ID card and an application form https://www.mofa.gov.vn/vnemb.china/zh/nr0507101868 are required.

Of course, you can also find an intermediary who will help you fill out the form, but only for a fee.

The consulate fee seems to be more than 500 yuan, which is a little more expensive than China passport. It usually takes five working days, not three. I wish you success!