That's how it has worked for me on the one time that I have used it. I had a Visa Electron card until recently so it wasn't an issue.

Bear in mind that the voucher will only be accepted if it's denominated in the currency that your flight is priced in, and while you can leave change on the voucher and use it another time, if the voucher doesn't cover the full cost of the flight you will be charged the full payment handling fee. Vouchers are available for a maximum of €500/£500 and the passenger whose name the voucher is issued in must travel.