HK TRANSIT VISA FOR VIETNAMESE CITIZEN

I AM A RETIRED AIRLINE EMPLOYEE FLYING ON A SPACE AVAILABLE STAND-BY STATUS. MY SPOUSE IS A PERMANENT RESIDENT ON THE USA WITH A VIETNAM PASSPORT. I BELIEVE SHE WILL NEED A VISA TO TRANSIT THE HKG AIRPORT. WHAT TYPE OF VISA DOES SHE NEED? WHAT FORMS? WHAT IS THE COST OF A DOUBLE ENTRY TRANSIT VISA? THANKS FOR YOUR HELP.