HEAT oven to 325ºF.
LINE 13x9-inch pan with foil. Spread 1/4 cup butter onto bottom of prepared pan. Mix dry gelatin mix and 3/4 cup sugar in medium bowl until blended. Add orange slices, in batches; toss to evenly coat. Arrange on bottom of prepared pan, overlapping if necessary to form even layer.
BEAT cream cheese, remaining butter, remaining sugar and vanilla in large bowl with mixer until blended. Add eggs, 1 at a time, mixing well after each addition. Gradually beat in flour. Pour over oranges. Place filled pan on baking sheet.
BAKE 1-1/4 hours or until toothpick inserted in center comes out clean. Cool 15 min. Invert cake onto platter; remove pan and foil. Cool cake completely.