Whip heavy cream in a chilled mixing bowl with a whisk attachment until soft peaks form. Slowly add sugar and vanilla.
Heat dessert sauce 15-30 seconds in the microwave, if it is not pourable. Make sure it does not get too hot. Fold into the whipped cream. Place in a covered container that can go into the freezer.
Place semifreddo in the freezer for at least 6 hours or overnight. Best if used within 3-4 days.