For a flight to Vietnam from London there is nothing direct. You will be flying in-direct to Vietnam, (Normally people travelling from The UK to Vietnam fly to Hanoi or Ho Chi Min City) and you will normally have to change planes in one of the larger Asian hubs such as Singapore or Hong Kong.
Top picks for flights to Vietnam : Qatar Airways (flights from Manchester too), Singapore Air or Cathay Pacific. Otherwise look at ebookers.com which covers multiple airlines and is sometimes cheaper than the airlines direct.
The airport code for Hanoi, Vietnam is HAN
The airport code for Ho Chi Min City is SGN
