I went to the counter today. The lady told me vouchers could only be used online. I kept saying I have been able to use them at the counter. Finally a manager walks near about something else and the lady quickly asks her and the manager says yes, you can use vouchers. It's at the end of the payment.

If that manager didn't walk by, there's a good chance the agent would have just said sorry, no, and told me wrong information.

It may be worth not mentioning the vouchers until after they go through the work of entering all the passenger info and ask for payment. That way they have invested time too.

The system automatically only applied the airfare portion of the voucher.

Note that birthdates and names also need to match with the vouchers. The lady had a typo and the voucher wouldn't apply. So they are locked to the passenger it seems.