Originally Posted by
36902BRF
Assuming the voucher is like the UK (Amex?) and US (Chase) ones then it is only valid for flights on BA metal. That is no code shares or partners. If you are in North America that means yes you have to fly to/through London because you have to fly BA metal and BA metal only flies from North America to London. So while the terms of the certificate (likely) don't state you have to fly to/through London effectively you do.
To answer your question on "best" use it is likely going to be in the F cabin or less so the J cabin. That said these days you may also want to look at purchasing a cheap cash fare and upgrading using points. Remember with BA you can only upgrade one class. Econ to Prem Econ, J to F, etc.
I usually see BA airfares going for about C$880 for roundtrip YVR-LHR. How much would it cost to upgrade to J? Furthermore, this is separate from issue of using the voucher right?