Did a Vietnam routing last year (for a week on the Côn Đảo Islands - beautiful place, the Six Senses Resort is highly recommended) for Mrs DrBenO's 30th Birthday.

BA LHR-HKG (J out, F Back) using GUF2 and AMEX 241, then plain Avios (CX J) HKG - SGN return. Then a short hop SGN - VCS on a Vietnam Airlines ATR: booked via the resort, only economy seating but access through the ticket to the (very pleasant) domestic lounge at SGN.

All-in amazing value and use of Avios (to me).