No Medallion window C+ (or domestic FC) upgrades on VS coded, DL flights. Like FC, C+ cabin is a separate fare class on DL (and not just an extra legroom coach seat) and the system can't process you into the DL SU comp upgrade C+ class as VS does not have an equivalent class (domestic FC upgrades likewise uses separate OU class). You may be able to get access to a C+ seat on day of departure, but most likely it won't be anything decent.