Place popcorn in well-greased pan.
In large heavy saucepan, melt butter; stir in sugar, syrup and salt. Bring to a boil.
Boil for 5 minutes over medium heat without stirring.
Remove from heat and stir in vanilla and soda.
Pour syrup over popcorn and stir well.
Bake for 1 hour, stirring every 15 minutes.
When cool separate kernels, store tightly covered.