Mix the milk, sugar, and vanilla together in one of the quart sized bags. Seal, removing as much air from the bag as possible. Place this bag inside the other quart-sized bag, remove air and seal (double bagging to prevent leaking and ensuring success).  Put the two bags inside the larger gallon sized bag along with the ice (we filled the bag with ice and then about 1/4 cup rock salt. Remove air and seal tightly.  Wrap the bag in the towel or put your gloves on, and shake, shake, shake.