I would like to know how to apply Hong Kong Visa for Vietnamese passport. Thanks!

You can submit your application to Chinese embassy in your country. If the embassy processes your application, you will wait for about four working days. If it is sent to the HK Immigration Department, you will wait for about one month. The required documents are your passport, filled HK visa application form (you can download it from the website of Chinese embassy in your country), a recent passport size photo and an invitation letter from your sponsor in HK.