Travelio offered a voucher instead of my refund. Do I have to accept?

No. Money owed to you is money, not credit toward another booking with the same company. Reviewers report being offered discount vouchers in place of a cash refund — that is not a refund, and accepting it locks you into transacting with them again while extinguishing your claim. Reply in writing that you require the refund to the original payment method, cite the amount and the date it was due, and set a deadline. If they hold the position, that written exchange becomes useful evidence at BPSK, because it shows the money was acknowledged as owed and simply not paid in a form you could use.