The DL website is, indeed, quite unclear. I've attached the screenshot from https://www.delta.com/content/www/en...-partners.html that should show your earnings based on fare class. You should be able to get an estimate from the DL website with regards to what you'd earn booking through DL - the MQMs will likely be the same (as the lowest they go is 100% regardless of fare class), but I would imagine that the base miles for discount economy would be different.

Ultimately you need to look at what the booking class on Virgin Atlantic is and on DL and see. Keep in mind that the base miles are a percentage of distance flown, and not ticket fare. Also keep in mind that you're only earning eligible on certain flights (i.e. the ones over the pond), and not any VS flight.