Yep, one issue. Perez packed a lot of story into those. As bluefall pointed out, it takes three times as long to read a Perez WW as it does a current JLA one. The opening narration of this one...
"What is the fate of the world?
"Some say the world is as it was, and as it ever shall be. Others say the world was born into darkness from chaos and shall die in the same manner returning to explosive Eternity.
"The poet writes that the world shall not end with a bang, but a whimper; the troubadour sings that the answer is blowing in the wind.
"'What can I do about it, anyway?' asks the common man with a shrug and a philosophical grin."
That's in the first panel.
I like to think the DCU and Vertigo are still the same Universe. The Zatanna that Paul Dini wrote for Vertigo, the one that used to be involved with Constantine and smoked joints, is the exact same Zatanna he later wrote in Detective Comics, after all. What happens in Vertigo is just the stuff the characters don't mention in front of the kids. :)