I suspect Delta may be able to sell you this routing if you call, though not guaranteed. I have noticed over the years that Delta.com has suppressed a lot of the more "adventurous" out-of-the-way routings that are out there and used to be offered (things that you once might have been able to buy on delta.com like PHL-ATL-MSP-SEA where there are plenty of one-stop options available), either because they assume customers don't want them or they want to limit clutter on the flight results pages or they want to limit MQM-seeking adventures or something like that. (Not that MQMs will matter any more soon.) Whereas Virgin may not have worried so much about those things, particularly the clutter one since they generally don't have many routing options for a given route.