Yes! if the voucher is less than the compensation amount you are entitled to. For example, if you are entitled to €600 and the airline gives you €250 voucher for that airline, you are still entitled to the remaining compensation of €350.
You may 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 the European Law EC 261/2004. That’s because you willingly gave up your reservation and entered into a new alternate agreement with the airline.