Spray a 1- 2 quart casserole dish (that will fit in your slow cooker) with non-stick cooking spray. Add chocolate chips, sweetened condensed milk, and vanilla in the pan and stir.
Place the casserole pan in the slow cooker and cover with the slow cooker lid.
Heat on low for 45 minutes.
Remove the casserole from the slow cooker and stir.
Option 1: Add alayer of marshmallows and return to slow cooker for about 20 minutes, or until marshmallows are slightly melted and gooey. OR
Option 2: Preheat broiler, top the melted chocolate mixture with the marshmallows, and broil the baking dish for 1 minute until the marshmallows are lightly golden brown on top.
Serve S'mores Dip with graham crackers.