See if they can see the Dulles flight on the 29th, which VS shows having availability. Or the VS flight to ORD on the 30th. Just make sure that any routing they give you can be replicated with a paid ticket, without paying a huge premium. They will courtesy hold the itinerary for a week or so, which will let you price out the various options for the paid ticket.

This is a risk that CO's availability doesn't exactly match VS, but it has usually been close enough for me. Sorry about that.