This is NUTS. If you want a flight to Vancouver from Toronto, Westjet has a sale now and glitch, whereby you'll have to buy TWO roundtrip tickets.
It's good for dates mostly mid-week, in May through October, but you'll need to buy a ticket from Toronto to Vancouver via a stopover in Montreal, then a return ticket direct to Toronto, but continuing on to Quebec City, ALL ON THE SAME DAY!
Click here on the Google Flights search to find flights to meet your vacation plans. Again, change your departure and return dates that are the SAME and simply don't use the return portion, unless of course you like flying a lot and hanging it in airports for FOUR hours!
You could always just buy one return ticket on the same day to go from Toronto to Quebec City on a whirlwind flight, leaving you stranded in La Belle Province! ;)