Preheat oven to 350º. Bake on buttered jelly-roll pan.
In medium saucepan, combine sugar, corn syrup, water and salt; cook over low heat, stirring constantly, until sugar dissolves; cook over medium heat to hard-crack stage (300ºF on candy thermometer).
Meanwhile, spread popcorn and nuts onto a buttered jely-roll pan.
Heat in 350º oven for 10 minutes.
Remove syrup from heat and quickly stir in butter and vanilla until butter melts.
Pour over popcorn mixture; stir to coat well.
Spread in thin layer on a flat surface. Cool and separate into clusters.
Note: Crystals formed when the syrup is cooking can ruin an otherwise perfect candy. Cover syrup with a lid for the first few minutes of cooking. The steam tends to dissolve crystals which might collect on the sides of the pan above the syrup.