Not sure what you mean by the "metro system"... Locally, Metro refers to the rail system in the city and suburbs. I suspect you are refering to Dulles airport's AeroTrain system that connects most of the building on the airport grounds.

Your flight from ORF can arrive in the A, C or D concourse. From A or C gates, you will take the Aerotrain to the Main Terminal (from D gates you still take a Mobile Lounge to the Main Terminal) where you will exit the secure area and claim your checked bags from UA on Baggage Carousel 1 (usually) or 5 (if 1 is broken). But you won't be able to re-check them with Virgin Atlantic for a few hours since their ticket counter (upstairs from Baggage Claim) doesn't open until about 3 hours before your departure. After re-checking bags you go back through TSA security and take the AeroTrain to the A Concourse where Virgin Atlantic uses gate A32.

A possible strategy to help fill the hours between flights at Dulles... Don't claim your checked bags from UA immediately on arrival - leave them unclaimed and pick them up a few hours later. In the meantime, take the 50 cents shuttle to the Udvar-Hazy Center of the National Air and Space Museum. Spend 2-3 hours there before returning to Dulles; pick up your checked bags from the UA; and continue as above.