According to their website, you cannot apply for a voucher if the ticket was bought with Miles+Bonus miles and Bank loyalty scheme points. See question 5 on Aegean's "Credit Voucher FAQs" page. I guess that means that if your flight is cancelled you can get an actual refund (of miles and cash) instead of a voucher. If you've had a ticket booked using miles cancelled, please let us know whether you got an actual refund.

For the time being, I don't see an option to use a voucher when booking through Miles+Bonus. There is a field for a "Promotional Code" when booking through the regular form (without Miles+Bonus), but this field does not appear when booking through Miles+Bonus.