Butter the sides of a large saucepan. In the sauce pan combine the sugar, water, salt, corn syrup and vinegar. Cook over medium heat to the hard ball stage 250 degrees F (120 degrees C). Stir in the vanilla and slowly pour the hot mixture over the popped popcorn, stirring just to mix well.
Butter hands lightly and shape into balls. Mixture will be hot so be careful. Place balls on waxed paper to cool.