Keep popcorn warm in 250°F. oven while preparing coating.  Melt butter in large saucepan; stir in sugar, corn syrup and salt.  Bring to boil; stir constantly.  Reduce heat to medium; boil without stirring for 5 minutes.  Remove from heat; quickly stir in vanilla and soda.  Place popcorn in large, heat-proof bowl; slowly pour syrup over popcorn while stirring.  Add pistachios; mix thoroughly.  Turn into greased 15¼  x 10¼ x ¾-inch baking pan; bake at 250°F. 1 hour, stirring every 15 minutes.  Remove from oven; cool.  Break into serving-sized pieces.  Store in tightly covered container.