Just because you're seeing open seats doesn't mean those seats are available. People often don't have seats assigned when booking, so they could be spoken for simply based on number of seats sold.

No reservation agent should be letting out the number of seats available on any flight, for security purposes and in case other airlines are checking routes.

Virgin America's standby policy for early departure does have a 2-hour window prior to departure. They also have confirmed standby, at the airport, for a fee, which bypasses the list and assures you a seat, if one's available. It's silly to ask a reservation agent for a guarantee on getting on a flight standby. It's never a guarantee, which may account for the hedging on the answer. The agents at the airport are in a much better position to give you assurances like that.