This article is from the Vertigo Comics FAQ, by with numerous contributions by Katie Schwarz kts@socrates.berkeley.edu others.
You can always start an argument over this. Officially, the books that
started out in the DC universe (most of them before the Vertigo imprint
existed) never left it -- that's SWAMP THING, HELLBLAZER, DOOM PATROL,
ANIMAL MAN, BLACK ORCHID, SANDMAN, SANDMAN MYSTERY THEATER, and THE BOOKS
OF MAGIC. The old issues where Morpheus met the Justice League, Swamp
Thing met Batman, etc., weren't erased, and there are still occasional
appearances of the new Dream in the Justice League, Swamp Thing at Hal
Jordan's funeral, and so on.
But in practice, of course, these books usually ignore the world of the
mainstream superheroes -- and lots of readers hate to impose nerdy
continuity between one book and another. It was definitely no favor to
Garth Ennis when the editors made him come up with some contrived
explanation about the difference between the Devil and "the First of the
Fallen" so that HELLBLAZER would be consistent with SANDMAN.
Neil Gaiman says: "I used to describe SANDMAN as being a long bus ride from
the world that Superman and Batman live in; these days it's probably a long
plane ride, but obviously Superman exists somewhere in the same world as
Tim Hunter et al. ... But it's now a lot harder to cross characters over
between DCU and Vertigo. Permissions need to be sought and so forth. There
obviously isn't a 'Vertigo Universe', or if there is it's just a subset of
the giant DC universe. ... Nobody left the DC universe (as a big concept,
not as defined as 'those comics edited by Mike Carlin') when Vertigo was
started; SANDMAN 5, SWAMP THING, ANIMAL MAN 1-20, didn't un-happen."
SHADE's presence in the DC universe was argued over; John Constantine
appeared, but then they apparently destroyed San Francisco and nobody else
noticed. Michael Blakeman Cleveland notes, "Spectre #50 adds another piece
of evidence that Shade was in DCU continuity because the American Scream
(the antagonist from Shade #1-18, and occasional later issues) shows up."
Most of post-1993 Vertigo (INVISIBLES, PREACHER, etc.) is creator-owned
and outside of the DC universe.
 
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