Add two types of vine to the plant class: a thief vine, which steals items from you, and a warrior vine, which is only found in bones files near the pile, and which starts with some weapons taken from the bones pile, which it uses to attack you with. (It would generate just after loading the rest of the bones level).

Thief vines, if adjacent to other thief vines, should pass stolen items away from the player, playing keep-away with them.