[[WMG: The vine is a supernatural being]]
The vine isn't just some crazy hyper-evolved plant, it's actually some kind of EldritchAbomination. 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.
* The vine creature is far more then a simple plant - it can grow in complete darkness, it makes sounds with its flowers, it grows directly into blood vessels...none of which a real plant could do. So you're dead on.
* 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. That would make it a kind of combination EldritchLocation and evil GeniusLoci.
[[WMG: 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.
[[WMG: 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?
[[WMG: The Mayans have a symbiotic relationship with the vine creature]]
One possibility is that the Mayans keep the vine creature in place in part by feeding it tourists. This doesn't seem too likely - the Mayans try to warn the party away from the overgrown temple before they touch the vines, and thus seal their fate. About the only real proof is that they don't MercyKill the entire party the moment they touch the vines; but if they did that, there wouldn't be much of a movie.
[[WMG: The vine is a normal vine that gives off a drug]]
At several points in the movie, parts of the background and people's faces ripple as if in a wind. Is it possible that the tourists are hallucinating the whole thing and/or having a psychotic breakdown?
[[WMG: The vine is really just a plant.]]
The vine is a member of a species that followed an exceptionally unlikely evolutionary path, evolving the ability to consume animals and possibly other plants so that it could survive in the caves below the Mexican rainforest after having its original above-ground niche occupied by a more successful competitor. Its normal prey items are bats and other cave dwelling animals, and it has developed the ability to imitate the sounds of its prey. Signs of intelligence in the book and movie are just random behavior that seems actively malicious.
[[WMG: The vine killed the dinosaurs.]]
The vine came down with the asteroid that made the Chicxulub crater on the Yucatan Peninsula. In the long impact winter following the event, it crept out across the planet and consumed almost all large lifeforms on the planet before starving out, leaving only a handful of dormant strands underground in the area surrounding the initial impact. A mining company (in the book) or a native society (in the movie) dug into the cave where it lay dormant, and the people living in the nearby village endeavored to keep it trapped.
[[WMG: The Mayans don't tell anyone about the vine because they know ''exactly'' what would happen]]
Realistically, if an intelligent carnivorous plant were discovered in the remote jungles of Mexico, it would be the scientific find of the century. There's no way in hell that it would just be destroyed, especially if it wasn't immediately clear that it was dangerous. Attempts to study it would inevitably allow it to spread, and by the time that outsiders figured out how deadly it really was, it would be too late to keep it from becoming invasive in every climate where it could live. After one growing season of man-eating kudzu, the world would be a very different place. Pretty much the only way to stop its spread is to keep information about it restricted, and to kill anyone who contacts it.