Preheat oven to 250 degrees.  In a very large bowl, place the popped popcorn kernels, with all seeds removed.

In a medium saucepan, melt the margarine then add the brown sugar, corn syrup, and salt.  Bring to a boil.

Remove from the heat and add the baking soda and vanilla.  Mix well and allow the mixture to bubble up before carefully pouring it onto the popcorn.  Gently mix the caramel and popcorn together until all the kernels are covered.

Spread onto greased cookie sheets and bake in the oven for 1 hour, stirring the popcorn every 15 minutes.  Allow to cool before storing in an airtight container.