The problem is your AC $275 voucher is only good for AC operated flights (if it's the usual MCO).

Best for you to do would be:

YUL-BNA-YUL using your AC voucher. Then nest a BNA-LAX-BNA on UA inside. I see that there's a T class fare for around $300 CAD base.

Not sure how you can get around claiming expense for this if it's 2 seperate tickets.