Given Vietnam Air is a member of Skyteam, their natural European hub is presumably Paris. This is reinforced by the fact France ruled Vietnam as a colony for many years, meaning trade and cultural ties are present along with a large Vietnamese community in France.
It would seem a little perverse for passengers outside the UK or Ireland to choose to connect in London unless said passengers have a love of backtracking. I don't therefore see what Heathrow really adds to Vietnam Airlines that Gatwick cannot provide, apart from higher costs and the magic prestige dust that comes with flying to Heathrow.
Anyone able to counter this opinion ?