In small sauce pan, bring water, margarine, and brown sugar to a boil on med-low to low. Cover and let boil 2-3 minutes. (Meanwhile, pop popcorn). Remove from heat, add vanilla, and let cool just SLIGHTLY (a minute or so), stirring constantly (it will thicken slightly, and won't melt the popcorn when you pour it on). Pour popped popcorn into a big mixing bowl; pour caramel sauce over popcorn, and fold in with spatula or wooden spoon until popcorn is evenly coated. Let cool slightly or completely if you want it crunchy. Enjoy!