A voucher only replaces cash if you signed for it. Usually you did not.
Check your flightProbably yes, you can still claim. A voucher only replaces your cash compensation if you agreed to that in writing. Article 7(3) is explicit: compensation is paid in cash, by bank transfer or cheque, and can only be paid in travel vouchers "with the signed agreement of the passenger".
Tapping accept on an app, or taking a voucher handed to you at a desk, is usually not a signed agreement to give up a legal entitlement you were never told about.
A meal or refreshment voucher. Nothing to do with compensation at all. That is duty of care under Article 9, owed to you separately, and owed even when no compensation is due. Accepting it gives up nothing.
A goodwill or apology voucher. Often offered quickly after a disruption, usually worth far less than the compensation. Unless you were told in plain terms that it was instead of your Article 7 compensation and you agreed to that, your claim should survive.
A refund voucher for the ticket. This replaces your fare, not your compensation. They are different entitlements and one does not cancel the other. Airlines pushed these heavily during mass disruption, and passengers often do not realise the compensation claim is still sitting there untouched.
A settlement you signed. If you signed something saying you accepted the voucher in full and final settlement, that is harder. Not always fatal, particularly if the amount was derisory or you were not told what you were giving up, but you would need advice.
Claim anyway, and say in the letter that you did not agree to accept a voucher in place of compensation under Article 7(3). Make them produce the signed agreement if they want to rely on one.
If they argue the voucher settled it, ask them for a copy of what you signed and the exact wording you were shown. In a lot of cases nothing of the sort exists.
The claim letter already includes the line refusing vouchers, which is the reason it is in there.
Check what you are owed in cash