What kind of aircraft was that when you booked? It's now showing as A330, and completely booked in all classes. If it was downgauged from an A346, it's quite possible the cabin is overbooked (the entire flight is probably overbooked, but that's normally less of an issue).
There also happens to be very limited availability in business class on other flights that day.
You might be in a bit of a tough spot as your ticket was issued by Delta and so Virgin is unlikely to do anything if you were to call them - and both they and DL will tell you you're confirmed in business class. They are probably hoping for cancellations between now and then, or they'll have to ask for volunteers for downgrade and/or to take a different flight. There may not be a whole lot you can do proactively, but I would definitely look into an ExpertFlyer seat alert in case any seats do become available on your flight.