Place popped popcorn in a very large bowl (sort through and take out un-popped kernels).
In a medium sauce pan, combine remaining ingredients except vanilla.
NOTE: Vanilla will cause the syrup to expand when added. Make sure the pan is at least twice as deep as the syrup before adding the vanilla.
Cook uncovered, stirring occasionally, until soft-crack stage (270 degrees on a candy thermometer).
NOTE: Cook over low - medium heat to prevent from burning.
Remove from heat and stir in vanilla. Slowly pour syrup over popcorn and toss until coated.
With buttered fingers, quickly shape into balls and set on waxed paper to cool and set up. Don't tightly pack the balls or they will be very hard.