Hong Kong Transit visa for a Vietnamese passport holder

Hi there, I hold a South African passport and my Vietnamese wife ( Vietnamese passport holder ) and i will be travelling from Japan to South Africa, via Hong Kong. We have an 8 hour transit in HK and would like to do a bit of sightseeing during that time. Can my wife apply for a transit permit upon entry at the airport in Hong Kong or do we need to apply for it at the nearest consulate in Japan?
We are dreading to apply for a tourist entry permit for her, knowing that we will only just spend a quarter of a day in HK.