It depends. If your fare is considered an "Internet-only" fare (often also called "Web-only" fares) and you couldn't have gotten the same rate by calling the airline's reservation number, you may not qualify for a voucher. If it's just a regular fare and you could get the same thing by booking over the phone, you probably can get a voucher if the fare goes down.
In my experience, however, most of the "Internet-only" fares are so darn low to begin with, that the voucher situation isn't that likely to occur.
Mary
MouseSavers.com