Melt butter, stir in sugar, syrup and salt. Bring to a boil, stirring. Quit stirring and boil for 5 minutes. Pour popcorn into flat pan. Take mixture off stove. Add soda and vanilla; stir well. Gradually pour over popcorn; mix to cover. Put in 200° to 250° oven for 1 hour, stirring every 15 minutes. Take out of oven and pour on waxed paper to cool.