Do I need to get visa to visit some places in Hong Kong?

Yes, you need to get one. You can submit your application directly to Hong Kong Immigration Department by post or consult with the Chinese embassy in Vietnam whether it accepts the application for Hong Kong. The following materials are required: the application form ID 1003A affixed with a recent photograph, the photocopy of your passport, the photocopies of your financial standing proof (e.g. bank statements, tax receipts, saving acounts passbooks, etc.), transportation arrangement proof (flight itinerary), and phtocopy of your ticket to US.