How Do I Know What I Think Until I See What I Say?

I read The Ruins by Scott Smith today. It was one of the rave books of the summer, and has been on my list for a while. I don't know how I feel about it. On one hand, it was very compelling - I read the whole thing in about three hours, because I simply couldn't put it down - but on the other hand, when i finished it, I felt a little empty. I think that's because one of the elements of the plot just didn't synch up properly, to me. HERE BE SPOILERS: In particular, the Evil Vine of Doom (EVOD), while gripping during the first read-through when everything is new, just doesn't make much sense when you think about it afterwards. So, there's the carniverous, possibly intelligent, obviously scary EVOD thing, right, and our four heros stumble on to the hill where it lives and are trapped there. Okay, fine, that all makes sense in the little world of the novel. But if the EVOD just wants to eat them, then why does it wait so long, until they are injured? Why doesn'