Specific: Reminder for non-used voucher

Working with voucher codes implies that you can assign unique codes to contacts when needed in your marketing campaigns. An action list would be most suited to link voucher codes with contacts. Voucher codes are imported into an action list, including a field that holds the code’s purpose. E.g. discount for campaign A, gain extra loyalty points for campaign B, free gift for campaign C…
A specific Stored Procedure component can be used within different journeys to assign an available code to a contact based on its purpose (e.g. discount A):

If the assigned code can be used within a Campaign journey (e.g. a subscription form in Campaign), this can be flagged in the action list, e.g. by setting a USED Boolean field to 1.
If the assigned code can be used externally, e.g. on the website, the Campaign assigned codes can be exported to the external system. Which, in turn, indicates if the code has been used by updating the USED field.

To send a reminder if a code is not used for a specific purpose (e.g. CampaignA_discount10pct) you can create a selection based on the action list within the journey that sends the reminder.