To clarify, the destination is Vietnam, although undecided in which order to visit Hanoi and HCMC.

KUL does not seem to be an option, as this will be before the BA service starts (or resumes ?).

The issue with the BA / AA flights, was that the BA F check-in agents were adamant that they could not tag the bags through onto an AA flight on a separate PNR, which would have enabled me to use the quick bag drop at the US gateway once I had cleared local customs. It seems they were wrong (I thought I knew that at the time), but knowing they are wrong is no real help if they won't do it. From memory, I am not alone in having encountered this problem (I think I have seen other threads in which this has been discussed).

My preference would be not to overnight at the gateway if I can help it.