John Constantine's character was introduced in The Saga of Swamp Thing #37 (June 1985); the Vertigo imprint didn't start until 1993. Swamp Thing didn't start having the Vertigo label on the cover until issue #129, and Constantine's own title didn't start having the Vertigo label until Hellblazer #63.
Books like Swamp Thing and Hellblazer merely had a "Suggested for Mature Readers" on the cover prior to the Vertigo introduction.