Many plant creatures are either mindless consumers or sentient protectors of nature. While it can be fiercely territorial over its patch of swamp, it doesn't care about it on any spiritual level. Because its body is so gooey and saturated with water, the Vine Horror doesn't care about fire and is able to protect itself from most weapons.
The Vine Horror is capable of speech, albeit the somewhat esoteric Sylvan, and is capable of reason. Debate is discouraged, however.