The Ruins/WMG

The vine is a supernatural being
The vine isn't just some crazy hyper-evolved plant, it's actually some kind of crazy Eldritch Abomination. No mere plant could ever evolve the amazing abilities the vine displays, especially the understanding and use of languages it would only have heard spoken once or twice. Not to mention that most predators in nature are nowhere near as sadistic with their prey as the vine is with it's victims. It's clearly some kind of malevolent being.
 * I got the impression that that's what we're supposed to think, that the evil magic done with sacrifices on that ruin created or transformed the vine into what it is today.

The vine is an evolved carnivorous cannibal
The vine possibly fed off the remains and blood of the dead Mayan sacrifices of the pyramid and over time, it eventually absorbed all the nutrients and knowledge from the corpse which allowed itself to take over the pyramid. Over a period of a thousand years the combined knowledge of the dead and evolution made it a super efficent carnivore plant.

The Vine's main food source would be (and likely still is) birds
It likely evolved the ability to mimic bird calls, thus attracting them towards it. Of course, this makes the entire plot of the movie moot, but what else would the plant eat in the jungle besides tourists?