This is an H class fare, Saturday stay only required. Leave Vancouver sometime in the early AM on Saturday and you can easily make YUL (not sure about Ottawa side trip). If you can't, then there is the late Friday night flight which gets into Toronto early and you can plan your whole day to get to Montreal via Ottawa easily. Since you have 24 hours for connections on this international ticket, you could even visit Ottawa for the day.

Return the same route leaving LHR early PM arriving YUL 3:30 PM, leave for YOW after 6 PM arrive after 7 PM in YOW, leave for YYZ at 8 PM, arrive YYZ 9:15 PM, leave for YVR at 21:55 arrive just before midnight in YVR

No guarantees about snowstorms at this time of year though. I always book the most complicated and then if things look tough (weather), I can short-circuit any routing if it looks like I will miss a connection. With SE status, I have never been denied my upgrade (but I won't guarantee for others).

I have never paid a change fee for this but things might have changed . . . You will be guaranteed a change fee if you try to split a segment though.

Still not all the miles you need but pretty close and I am sure you can find something creative and cheap before Dec 31 on a Star carrier.