Avion does not want to do this or know that they can do this....
You should be able to call avion and say, I have a problem, I want to book using avion but I can't and need to do it through the airline. (Perhaps telling them you have a voucher you want to use with the airline will work). This give you some ID number for the transaction.
You then book the flight through the WestJet call centre or website. Billed to your RBC account.
You then call avion back. They issue a credit against the WestJet charge on your credit card.
I have been successful in doing this when the link between the avion portal and Air Canada portal have been down. Others have been successful in doing this when they have wanted to get certain fare buckets with Air Canada that were not available in the avion portal. I have never tried it with WestJet.