The fall season at Off Broadway’s Flea Theater underscores the increasing importance of co-productions and partnerships amid a nonprofit landscape where funding can prove increasingly scarce.Season kicks off with a remount of “Invasion!,” Jonas Hassen Khemiri’s war-on-terror comedy that won an Obie when it was produced last winter by the Play Company. That org again presents the show, helmed by Erica Schmidt, in the first offering of a seasonlong Play Company residency at the Flea. Later in the fall, the Flea teams with Vampire Cowboys, the downtown company with a comicbook aesthetic, to produce the world preem of “She Kills Monsters” by scribe Qui Nguyen and helmer Robert Ross Parker. “Monsters” reps one of two partnerships for Vampire Cowboys next season, with the company also teaming with Ma-Yi Theater for a spring 2012 remount of VC play “The Inexplicable Redemption of Agent G.” Also on tap at the Flea is a new project from downtown vet Elizabeth Swados, whose “Kaspar Hauser” attracted attention there a couple of seasons ago. During the 2011-12 season the Flea’s resident company of actors, the Bats, have scored coin from the National Endowment for the Arts for the first time. Off Broadway partnerships have grown more common in recent years. Examples last season include the New York bow of Tony Kushner play “The Intelligent Homosexual’s Guide to Capitalism and Socialism With a Key to the Scriptures,” co-produced by Signature Theater and the Public.