Nope.

When you use RBC Avion points, you're essentially buying your flight through a travel agent (in this case, RBC -- or whoever they contract with to fulfill their points "purchases").

Companion awards, along with other WestJet Rewards forms of payment (like WS Dollars) can only be used when you're booking directly with WestJet.

RBC won't/can't accept a WestJet companion voucher as payment, just like WestJet can't accept Avion points when you're buying from WestJet.