In saucepan, over low heat, combine peanut butter chips, milk, and a 1/2 cup of sweetened condensed milk. Cook, stirring constantly, until thick and smooth.
Remove pan from heat and let cool. In bowl, w/ mixer on medium speed, beat cream cheese until fluffy. Beat in remaining condensed milk and vanilla.
Spoon in chopped peanut butter cups. Add whipped topping to the cooled peanut butter/milk mix. Pour peanut butter mix and cream cheese mix, alternately into pie crust, ending w/ cream cheese mix. Cover w/ plastic wrap and freeze until firm, about 1 hr. Let stand at room temp for 10 mins, before serving.