Well, I'm used to looking at that from the US, where flying directly to Vietnam was impossible until United introduced a new direct flight.
Anyway, I digress...
You can pretty much fly into anywhere in Southeast Asia (or east asia) and then fly into Vietnam - see if that'd be cheaper. For instance, many flights from the US currently connect there via Taipei, Hong Kong, Bangkok, and Singapore. So you might want to look at hopping from the UK to one of those cities first... then find a lower-cost regional flight.