transit visa for Vietnam

hi Could you tell me if i need to apply for a visa for Hong Kong as we transfer to another flight onroute to vietnam but don't leave the airport. Also I was told by the travel agent when i booked my flight that we didn't need a visa for Vietnam is this true? If we do need to apply for a visa when do you suggest we do we arrive at Hoi Chi Minh on 18th October and is the visa on arrivel reliable or are we better applying via embassy in UK? Thanks Liz