I can't articulate why this annoys me so much, but it does for some reason. Maybe because this sort of editorial nonsense was kinda sorta used to justify killing Hellblazer. John Constantine was always a part of the DCU, as was Animal Man, Doom Patrol, Swamp Thing etc etc. They were seperated editorially from everybody else, but unless someone can point to something I am not aware of, there was never a seperate Vertigo-verse until it was decreed to be so in Flashpoint and then undone one second later.
Swamp Thing of course has fought Batman, John has met the Phantom Stranger, Doom Patrol went from being a Vertigo title to a non-Vertigo title and kept their Vertigo continuity. And of course Superman, Batman, and Constantine all have little cameos in Sandman, not to mention Daniel showing up in Morrison's JLA and Luthor meeting Death.
Of course there were some inconsistencies, but no more so than Garth Ennis writing Punisheras a Vietnam vet while everyone else in the world is on a sliding timeline, or even just the fact that Superman doesn't just zip over and capture the Joker every time he breaks out of prison.