I would seriously advise not to accept such vouchers.
The voucher can only redeemed with the issueing airline.
These vouchers are useless, if you catch a good on a codeshare (operated by the issueing airline, but sold by another airline) or if a consolidator (via an OTA) sells a good fare. Airline-owned booking sites do not see all available fares (which a Kayak meta search machine can see).
It's not 100% clear whether a phone booking will be waived if the voucher cannot be redeemed online.
A EUR 600 cash payment does not expire, can be booked for other people, can be used for other airlines (and codeshares) ..... etc etc