To repeat a bit, my understanding of the EU 261 rules is that (1) if OP has a ticket receipt for a flight operated by a EU carrier, there are rights and (2) compensation to the passenger comes from the operating carrier, which is then free to pursue reimbursement from it's codesahre partner. OP might feel that DL should pay because the situation seems to be DL's fault, but this might be counterproductive in terms of pursuing legally mandated compensation.

DL might send some apology miles in addition and/or a voucher for the fare difference between Premium Economy and coach fares, but DL is unlikely to pay the OP for pain and suffering or to give two GUCs to a PM when the downgrade was only one flight and only from Premium Economy to coach.

It sounds to me like the schedule change was never connected to the OP's itinerary, although checking the flight on the operating carrier's website--for instance, while watching seat assignments--would have alerted the OP to the problem when it would ahve been easier to find a better solution. I suspect that when the OP talked with DL, the DL agents were only looking at the OP's itinerary and not the schedule, so it looked like the VS flight was fine.