You obtain vortex after you beat the game initially.
Usually if the Vortex doesn’t have any bad guys to suck up it will shrink and then disappear. However if you cast a rock into the vortex it will replenish it which can keep it from disappearing until you can get a bad guy to get sucked up. Shooting it with bullets (using the Machine Gun) or mines also replenishes the vortex.
Vortex is amazing in Challenge mode. Just lay one of these near an enemy entrance and watch them disappear :P.