Val Gallaher largely agrees with David Bird here, in that Vertigo was in decline before Didio took over and that it was largely due to conservative book selection.
Her argument is fairly absurd. Even if you buy that there was some kind of creative rut at Vertigo around 2001, that doesn't explain Vertigo's state in 2013, especially considering that in between it published brilliant, creative books like Fables, Scalped, DMZ, Northlanders, Unknown Soldier, Unwritten, Madame Xanadu and many more.
No, Vertigo's sad state was caused by three things:
1) The DCU taking away all company owned characters.
2) The drastic reduction in output.
3) A change in the contract structure for creator owned books, that drove creators like BKV away.
All three of these things happened within one year of Dan Didio becoming publisher. You want to know who killed Vertigo? Look for the guy with the bloody knife.