Was the DL agent aware that you had a FC ticket? Perhaps he/she didn't notice the FC boarding passes or assumed that you had received free elite upgrades (in which case, rebooking in coach would have been SOP unless something otherwise had been negotiated as part of the VDB). I'm almost surprised that DL would ask a (paid) FC cusstomer to take a VDB unless there was a problem of being oversold in FC. I would assume that DL policy is to rebook a paid FC VDB into FC whenever F > 0 and otherwise to explicitly tell the passenger that the new flight will be coach and get agreement that this is understood and accepted as part of the VDB.

When you noticed the coach seat on your new boarding pass for tomorrow, did you try to ask the agent? Have you phoned DL about this if no agent is available? Waiting to fix it means that it is less likely that there will be a FC seat available on your new flight. How would you feel about staying in coach and getting a refund of the fare difference in addition to the $400 for the VDB? (Would you prefer this additional money to getting FC seat for the new flight?) However, be careful about how the fare difference will be calculated; it can amount to nothing.