VS and DL have exact same ZN1XU6D4 fare filiings for $3129 base and $1106 YR. Sounds like DL just doesn't have Z bucket available in one direction (did you check?). Doesn't necessarily mean there's no coordination going on just because they don't have exact same inventory available on every flight. VS does have a slightly lower ZN1XU6D1 filing ($100 less) for which DL does not have a matching filing. This is the fare that you booked. Could be the DL has better brand recognition out of SFO and thus the $100 premium.