Sometimes airlines offer vouchers but more often then not the voucher is less than the compensation amount you are entitled to. For example, if you are entitled to €600 and airline gives you a €250 voucher to be used with that airline, you are still entitled to the remaining compensation amount of €350. You can also choose to return the voucher and ask for the full compensation amount in cash.
If you were on a flight that was overbooked and the airline offered you a voucher and a later flight in exchange for your reservation, and you accepted, you are not entitled to compensation according to EC 261/2004. That's because you willingly gave up your reservation and entered into a new agreement with the airline.