I did on one occasion have a cancelled flight credit that I needed to use. As I famously cannot call, what I did was book the itinerary I wanted, and then communicated with AC over Twitter DM providing both PNRs, and they replaced that booking with the one using the cancelled flight, and I went back in and took the same seats. It took, like seven transactions to credit all the purchases because all the taxes are all separate and they double refunded one and had to charge it to me two months later when they figured it out. (And I had a bear of an expense report to do, twice.) But it all worked out. I would not go out of my way to get a voucher if I could have a refund, but it would; not be the end of the world.