Pretty easy: Use the "Multi-City" option on aircanada.com and pick a city on the way from YVR to YYZ, ie YOW, YYC or YEG. It will show you the various flights from one city to the other and if your look carefully at the displayed fares you should still be pretty close to your original fare. YOW should actually work out well, since you will benefit from the 500 miles minimum on YOW-YYZ.

This should work for YVR-YYC-LAX as well.