Combine sugar, butter, corn syrup, water and salt in a large saucepan over medium heat. Bring it up to about 265-275°F, or until hard. Use a candy thermometer. If you don't have one, drip the candy into a small glass of cold water. If the candy forms a very hard ball, it's ready. Make sure mixture doesn't boil over.
While candy cooks, pop both bags of popcorn and spread about 2 quarts or 1 1/3 bags of popcorn on one large or two small cookie sheets.
Put the popcorn in your oven set on its lowest temperature. This will keep the popcorn hot so that the candy will coat better.
When your candy is hard, add the vanilla.
Pull the popcorn from the oven and quickly pour the candy over the popcorn in thin streams.
Mix the popcorn so that each kernel is coated with candy, put the popcorn back into the oven for five more minutes, then stir once again. This will help to coat each kernel. You can repeat this step once more if necessary to get all of the popcorn coated.
Pour popcorn onto a large sheet of wax paper to cool. Watch out of the heat.
When popcorn is cool, break it up and immediately put it into a tightly sealed container, such as Tupperware. This will ensure that it stays fresh. It will become stale rather quickly if left in moist climate.