Originally Posted by pauldb

You can book the outbound online as a one-way with the voucher. You only need to speak to BA about the inbound. You can either ring at midnight (or 1am BST) and have it added to the outbound booking, or you can book it online standalone (for full 2x avios, if you have them) then call them to have it "vouchered" to get half the avios back.

One voucher will do your whole trip, and actually the open-jaw aspect makes no practical difference here.