Virgin Atlantic (VS) and DL don't yet have their joint venture approved, so they can't yet legally collude on pricing and inventory. Therefore, their flights are subject to their own fare rules and availability, which aren't coordinated.

I would expect this to change once the joint venture receives approval and is implemented.

(The DL corporate purchase of 49% of Virgin Atlantic, the codesharing agreement, and the reciprocal frequent flyer recognition is what has been completed; that's separate from the anti-trust immunity and joint venture.)