1. Melt butter in a medium saucepan over medium-high heat. Add onion and sprinkle with salt; cook, stirring frequently, until soft (about 7-9 minutes). Add vinegar, barbecue sauce, and 1/2 cup water; simmer 5 minutes. Add turkey to sauce mixture, and stir until thoroughly heated (about 3-5 minutes). Add pepper, taste, and season with additional salt, if needed.
2. Arrange turkey evenly on bottom halves of buns; top each portion with additional sauce. Cover with bun tops; serve with pickles and hot sauce, if desired.