In a mixing bowl, combine 1 1/2 cups of graham cracker crumbs and the butter or margarine with your hands. Flatten out crust on the bottom of the springform pan.
In a large bowl, beat softened cream cheese until smooth. Beat in sweetened condensed milk and mix until combined well. Add the eggs, sour cream and vanilla, and continue to beat. Mix until very smooth, and pour mixture into the springform pan. Sprinkle the remaining 1/2 cup graham cracker crumbs over the top of the batter.
Bake for 45 to 50 minutes. Let stand and cool. Refrigerate to chill.