The Op didn't mention having AA or BA miles to fly on CX, so I would think something on Star Alliance will be the best option especially with UA mile redemption as you can book oneway. If the Op is solidly sure that there won't be any need to change the itinerary, then US mile redemption is the cheapest. Best option for a direct flight from Vancouver to Hong Kong on *A is with Air Canada. Plus availability with *A will probably be better as they have more Asian partners than anyone else for flying to HK.