I hope you get your bags back.

As a Canadian, I can tell you that getting the bags through the customs part without your being there can be a bit problematic and take time. I'm guessing your bag is probably held up somewhere within this process. Unfortunately, AC doesn't scan bags as they get on and off aircraft (or, at least, they don't make that info. available), so while UA can tell you the bag got to YUL, you won't know which flight it gets on to YVR until it arrives.