I haven't used a VDB voucher this year, so things may have changed, but in past years time and date changes were free -- you only run into the $100 change fee if you change the routing. So, if there's a place you know you want to go, but aren't sure when, you can book a decent routing for some random set of dates with QVW availability and then switch things around as plans firm up (or as more desirable flights open up).

Dan