Few questions:
1. You booked a single itinerary HKG-YVR-SEA with Hong Kong Airlines ? Who's operating the YVR-SEA leg ?
2. Which country's passport do you hold ?
3. When you say "Internal Transfer" you mean International to International @ Vancouver, correct ?
Edit:
- I see that Hong Kong airlines is not listed as a "International to US" transit guide
http://www.yvr.ca/en/passengers/navi...ssenger-guides
- I tried a similar booking up to point of payment with Indian identity and the website never warned me of Canadian Visa requirements