It depends on what type of voucher it is
if it is : you volunteered to miss a flight and got a generic $XXX voucher good for a year on any anything, then yes, you can use it for vacation.
If this is a : you canceled a flight and they gave you the refund (or excess refund on a canceled flight) , then no, it can only be used for airfare
if it is a : return on funds because the flight went down in cost significantly enough you got extra funds after your change fee and they issued you a voucher, then no, it can only be used for airfare.
no matter what, it is only going to be usable through the company (delta) and not a 3rd party flying delta or a TA.