I am a US Green Card holder & want to travel to HK and then Macau. Do I need to apply for both visa?

could you please tell me which country's passport are you holding?

I have a Vietnam passport

If so, you need to apply the entry permits for both Hong Kong and Macau at the consulate or embassy in the US.

Documents you need to prepare for HK entry application are: an application form (ID 1003A), your recent photograph, photocopy of your passport, photocopy of your financial standing proof, e.g. bank statements, savings accounts passbooks, tax receipts, etc. and proof of employment (if any), e.g. company leave letter, salary slips, etc., photocopy of proof of transportation arrangement, e.g. flight itinerary, and photocopy of proof of the proposed leisure visit to Hong Kong, e.g. receipt of joining a package tour to Hong Kong, itinerary, etc.

For Macao entry application, the application document requirements are basically the same with those for HK.

Thank you very much for the information.