I'm vietnam citizen, how can I obtain a HK visa at the airport?

You cannot use mainland PRC entry permit to enter HK; you should apply for another entry permit to go there and you cannot get it at the airport. You should apply for it in advance.
If you go to HK for sightseeing purpose, you should apply for a tourist entry permit. You need to prepare the required documents at first: copy of your passport, the completed application form affixed with a passport-style photo, proof of your financial support, transportation arrangement, and your travel itinerary. And then apply for it at the embassy of PRC.