Melt butter in large saucepan over medium heat. Stir in brown sugar and vanilla. Mix in the oats. Cook over low heat 2 to 3 minutes, or until ingredients are well blended. Press half of mixture into the bottom of the prepared pan. Reserve the other half for topping.
Meanwhile, melt chocolate chips and peanut butter in a small heavy saucepan over low heat, stirring frequently until smooth. Pour the chocolate mixture over the crust in the pan, and spread evenly with a knife or the back of a spoon.
Crumble the remaining oat mixture over the chocolate layer, pressing in gently. Cover, and refrigerate 2 to 3 hours or overnight. Bring to room temperature before cutting into bars.