If it's a Delta PNR then they are the ticketing carrier, and VS is the operating carrier. The changes would need to be done with DL inline with their rules.

I've never tried buying a cash upgrade at the airport when it wasn't ticketed by VS themselves. I suspect it can be done, so it's worth asking.

Remember that VS Premium is still pretty good and if you've been used to DL Comfort+ then VS Premium is a big step up.