Hi Dorothyr,
I think Raffles put it better, but here goes anyway,
Originally Posted by dorothyr
- obviously, with bmi not flying to Tokyo, I'll be using another carrier. I just checked Expedia and noticed that both LH and SAS do one-stops (I'll be coming from EDI or possibly GLA); would bmi give me a choice of carriers, or is it a case of whoever has seats available? Are their business class amenities roughly equivalent? - if I'm splurging I want to enjoy myself as much as possible.

Generally it is best to decided which carrier you want to fly (assuming they fly to your destination) and request them. If you just phone DC and ask for a redemption from point A to B, they'll put you on whoever, but there will be options for you to choose from if you do your homework first. Note that the routing must be one of the more direct routes (e.g. You could request to fly a carrier from UK-Japan via Korea, but not UK-Japan via South Africa).
- if I want to include a brief stopover in Ho Chi Minh City, is this possible? Would I need to cash in extra miles? Or would it be easier to book the Tokyo flight and hop from there to HCMC and back again on a separate ticket?
If you wanted a stopover, the routing/flight would have to go via that point, because award redemptions must be via the most direct routing. I don't know of any direct *A routings that go from the UK to Japan via SGN. You may however be able to fly via BKK or ICN or somewhere else nearby. You could then stopover here and add a side trip to SGN. That might be cheaper (both in terms of cash or miles) than flying from Japan to SGN.