I suspect using BA via Europe will be very expensive (see the existing surcharge thread for BA), not to mention using AA miles I'd guess it will probably take two awards - YVR-LHR and LHR-HKG. But I suspect you are using BA to find seat availability on CX? (I was going to say something like Cathy is pretty shyk but... )

OTOH, using CX and some flexibility, YVR may be the easiest North America destination to find award seats to HKG from. Which class of service are you looking for?