Grease a baking sheet with butter and set aside.
Put the popcorn in a large roasting pan and keep it warm while you make the molasses bacon coating.
In a heavy saucepan over medium-low heat, stir together the sugar, molasses, 1 cup water, the salt, and vinegar. Cook, stirring constantly, until the mixture begins to boil and reaches 250 F when tested with a candy thermometer (this is the hard ball stage). Remove from the heat and stir in the butter, vanilla, and bacon.
Slowly pour the mixture over the warm popcorn and mix well, using a wooden spoon. When the popcorn has cooled just enough to handle, butter your hands well and make baseball-sized balls by pressing the mixture together firmly, but gently enough not to crush the popcorn.
Put the warm popcorn balls onto the buttered baking sheet and let them cool until they have hardened. To store, wrap them individually in plastic wrap. Popcorn balls will keep for 5 to 7 days unrefrigerated.