Originally Posted by
UKtravelbear
the cost of the sector downgraded
Quite right. I wasn't quite clear enough.
As for miles, if you book a VS flight using DL (or some other kind of partner) miles, I don't believe that VS are able to "refund" a partner airline's mileage currency. In that case, if you reject receiving VS miles (a simple "I don't collect those" should suffice), then you can put it back on the airline to come up with another option.
In general, EU rules make it cheapest for the airline to downgrade passengers travelling on miles, so one needs to be aware of that before pushing back too hard. But IMO I would only volunteer if I was getting something more tangible than 100K VS miles...