In a saucepan, melt the margarine over medium-low heat. Add the marshmallows and let sit for about 30 seconds, turning the heat to low. Gently stir. Add the vanilla extract and lime gelatin and stir again. Let the marshmallows partially melt but remove from heat before fully melted. Stir (off the burner) until completely melted.
Add the Rice Krispies and mix until combined.
Grease a 9x13x2″ baking dish and spoon the Rice Krispies Treats evenly into the pan.
Pat down gently with a greased hand, if necessary.
Let sit until cooled and firm, cut into squares, and serve.