devtrackers.gg
Looking into the event, the automated conversation for unused Vouchers was 1:150,000 Credits. This initially surprised me as I expected it would convert to Credits at the "buy Voucher" rate, but it was pointed out that could allow for people to buy Vouchers with non-Credit resources and then wait out the time period for a non-Discounted exchange to Credits. That would be a subversion of the coversion-rate concept, so Vouchers were converted at the output rate, not input rate.
I would suggest a CS Ticket. Customer Support governs account interactions and would be able to see what currency was used to purchase the Vouchers that weren't used. Having spare vouchers isn't likely a widespread issue, so they might be able to refund the "buy Voucher" amounts/types that you used for the vouchers you didn't spend.