We're doing a Vietnam trip this summer (not in J, as family coming too) but did have a similar 'which option should we chose?' discussion. Our thoughts were that we wanted a reasonably linear journey within the region - to cut down travelling time (and hence time 'wasted' in airports when you could be seeing more interesting things)

For example we contemplated LON-BKK then a hop to Siem Reap, then Phnom Penh, then Ho Chi Minh, City, then Hoi An, then Hanoi. And then back HKG-LON. Not all by plane! one LONG journey South East and then North-wards with no real doubling back. In the end we decided to tour Vietnam only and got QR flights into SGN, a cheap flight from HAN to SIN (as [MENTION=3509]MikeFly[/MENTION] suggests) and back to LON from there.

As a result, have a think about open jaw options (even though the FT community advice is to avoid the BA LHR-BKK option!) which would be fine with your 2-4-1 given the distances involved.