flyerCO has it correct. The best part about VS is the lounge, and as D1 pax, you'll have access to the VS lounge at LHR. About the only thing you'll miss by not flying VS is the purple mood lighting. Take the DL flights and don't worry.

Also, if the VS arrivals lounge is open when you get to LHR, that is a great amenity that you will be entitled to use as well. A cooked-to-order breakfast and a shave/shower/change of clothes is a great way to start the new day.