This same nonsense happened earlier this year with DL coded VS metal flights. DL appears to have had an error in their pricing rules which excluded DL flight numbers in the VS and AF codeshare range for certain W fares, since DL was using W for C+ and AF/VS were using W for PE.
With mine on VS, they tried the same nonsense and I spent an hour with DL getting them to fix it (had to take an earlier flight, fortunately this was NYC), and then called VS and made sure it was indeed fixed.
This definitely appears to be an elementary error from DL's rate group, because airlines preclude certain flight number ranges all the time, such as the JFK-LAX flights.