Actually, if you get rebooked on a VDB often you are rebooked into full Y and get more miles due to the 50% bonus EQM. In fact in October I was bumped PHX-MSP on an award ticket, was rerouted via DTW in Y class and earned over 7K banked miles
Nice! My experience with this has been hit and miss. I have gotten the COS bonus when standing by for an earlier flight, but have also not gotten it after being rebooked after a bump. Plus, I have no idea how DL would handle that!