In a large skillet over medium heat, heat the butter and olive oil until the butter melts completely. Add the onions and season with salt and the paprika. Cook, stirring occasionally, until the onions are browned and start to caramelize, about 15 to 20 minutes. Lower the heat and continue to cook, stirring occasionally, until the onions are browned and tender but not falling apart, 15 to 20 more minutes. Transfer the onions to a baking sheet to cool.
In the bowl of an electric mixer fitted with the paddle attachment, mix the sour cream, cream cheese, mayonnaise and vinegar. When smooth, remove the bowl from the mixer and stir in three-quarters of the caramelized onions, three-quarters of the scallions and the cayenne. Taste and adjust the seasoning with salt.
Transfer the dip to a serving bowl and top with the remaining caramelized onions and scallions. Serve with bread.