hongkong visa for Vietnamese citizen in Seoul

Generally, these documents are needed for HK visa application:
round trip tickets to HK, hotel booking in HK, your itinerary, your bank statement with at least around 6 months' history, passport and application form with recent taken photos
Once you submit the documents, they will allow you take your passport home. Relax!

If the Chinese embassy can directly issue a HK visa, it takes 4 working days. If you are told that all the documents have to be sent back to HK Immigration Department, the processing takes 4-6 weeks.