There was a thread on this about a 6 weeks back (somewhere). I think the OP on that said taxes were not charged when redeeming on domestic. I was able to get a $36 refund on an international itinerary when I called the EXP desk, after being told by an airport agent that I had to pay full taxes. The EXP desk pretty much told me the airport agents are not trained to sort out the taxes, and it's best done by mail with the ticketing folks in Florida, when a VDB voucher is involved. Maybe someone will come along and post a link to the older discussion.