The old "guaranteed" seat thing was always smoke and mirrors. Other than in truly massive IRROPS, UA (and the other legacies) will all overbook by one more on a paid full Y ticket because even if that results in an oversale, the cost of a VDB is likely not that much and the risk of an IDB pretty slim.

Worth noting that VDB credits likely cost the carrier 70-75% of their face value and some industry analysts suggest that less than 50% of credits are ever used. That may mean a cost of $0.35 on the dollar. Or, a $600 VDB averages 210.