I think the difficulty is that while the GUC instructions/terms state that Z is needed for a DL marketed KLM operated flight, there isn't anything that says what the applicable inventory is for VS. If they have to convert to a VS flight number, then they need G... but they may be looking for OP which doesn't exist there.

With KLM fortunately both DL or KL numbers need Z to upgrade, so when they convert from DL->KL to reticket the same inventory is still there.