Some people use "return flight" to mean "a complete set of round trip flights" as distinguished from "a one way flight", which is my guess here.

I agree that Delta isn't obligated to and probably won't provide new flights for the old price using the credit this much later (they generally would if you were shifting a few days at the time of cancellation). I also agree with others here that accepting a credit/voucher when you're entitled to a refund is almost always a bad idea. Too much flexibility lost being locked in to one carrier (or even one use for your money that may be needed for other priorities)

If the OP explicitly requested the voucher, they're probably stuck with it. But if DL "automatically" generated it, or if an agent misled to thinking it was the only option, then it's acceptable to go back and ask now for the refund. I know they were auto-generating credits for no-shows on operating flights (where only a credit would be due), but I don't know if they did the same for flights they canceled.