Just remember that VDB is an art more than a science and is dependent on so many factors day-of-flight: weather, misconnects, whether a sudden influx or outgo of pax not figured in UA's historical data, etc., etc. We've all seen flights zeroed out that took no vol'rs and we've seen the opposite as well. While being cautiously optimistic, expect the unexpected; and also plan out your fallback position if VDBs are taken as sometimes, UA needs VDBs yet doesn't have quick and easy protection for volunteers.