Make the sauce: Combine the mayonnaise, ketchup and vinegar in a small bowl. Season with salt and set aside. Cover it and chill for at least 30 minutes. Make ahead: Sauce can be made, covered and chilled for up to 5 days.
Make the burger: Season the pork generously with salt and pepper. Form into 4 patties, each about ¾-inch thick. Refrigerate until ready to grill.
In a medium skillet, add the bacon and cook over medium heat until the bacon begins to crisp, about 4 to 5 minutes. Flip it over, and cook it for an additional 3 to 4 minutes. Using tongs, set the bacon aside until cooled slightly.
Prepare a charcoal or gas grill for medium-hot heat, or place a grill pan over medium-high heat. Place the patties on the grill or grill pan and cook until lightly charred on the bottom, about 4 to 5 minutes. Flip the burgers and top each patty with 1 slice of cheddar. Grill them to desired doneness, about 3 to 4 more minutes for medium. Grill the buns for 1 minute, until lightly charred.
Spread the sauce on the cut sides of the buns. Top with the burger patties, tomato and onion. Serve immediately.