Chop cranberries in food processor or blender until fine and no large pieces remain. Transfer to mixing bowl and stir sugar into cranberries. Cover and chill for at least 2 hours or up to overnight.
Drain any accumulated juices from cranberries and reserve. Stir in drained pineapple and marshmallows. Set aside.
In a mixing bowl, whip heavy cream adding powdered sugar and vanilla to sweeten until soft peaks form. Fold into cranberry mixture until fully incorporated.
Refrigerate or freeze. Garnish with pomegranate seeds.
