I suspect because of the distances involved that the flight to SGN (and then bus to Phnom Penh) may be best. Even if I can use KA as an airline with the "oneworld" plan to get to Phnom Penh, the AA miles I'll have to use would far outweigh what I'd need to use just to get to SGN.

The "oneworld" awards usage really seems suited for people with a ton of miles (I'm just pushing 100k).