My experience (as a DL Platinum) is to book the flight on Delta.com, then go to the Virgin website, you can now enter the DL confirmation number directly into the Virgin site, it will show the flight details then go to the seat selection tab and it will let you pick those seats at no charge. I often book with a companion who has no status, my seat selection is at no charge, and hers comes up with the appropriate seat selection charge for Virgin.

if you book online and for some inexplicable reason it all goes horribly wrong, you can cancel for free within the first 24 hours anyway, but don’t think of that, it works every time and it’s easy.