I'm Vietnamese, but now I am living and residing in the United States.

You need to apply for a tourist entry permit for HK ahead of your trip. The first thing is to prepare these materials: a completed application form with a passport-style photo attached, copy of your passport, your financial standing proof, hotel booking record, transportation arrangement, and your travel itinerary.
The next thing is to submit the application to the embassy of PRC to apply for it. If the embassy deals with your application, 4 working days are needed; but if it is transferred to HK immigration department, 4 to 6 weeks are needed.