If ever given the option between cash and a voucher, take cash! You can always spend cash back at the airline who gave it to you... Or at another airline... Or on anything else you want. There are no doubts with cash.
I've never personally had this problem on Frontier, but I once got a VDB on TWA for a "free ticket anywhere we fly in the CONUS." When I got the ticket there was a lot of fine print about what fare classes it could be used on. After a lot of fighting with TWA, I finally was able to use it over Memorial Day weekend.
Its very hard to find out all the T&C of a VDB voucher, especially when agents are trying to get a full flight away from the gate.