In a medium saucepan, heat 2 tablespoons of the oil over medium heat. Add the onion and cook, stirring occasionally, until caramelized, about 15 minutes. Add the garlic and cook for 5 minutes more; let cool.
In a medium bowl, stir together the sour cream, mayonnaise, milk and vinegar. Stir in the onion mixture and chives; season to taste with salt and pepper and refrigerate.
Heat the remaining oil in a medium, deep skillet fitted with a deep-frying thermometer until the oil registers 350 degrees . Meanwhile, peel the sweet potatoes and thinly slice them crosswise into thin rounds with a vegetable peeler. Rinse under cold running water and pat dry.
Fry the sweet potatoes in 5 or 6 batches, turning frequently with tongs, until crisp, about 3 minutes per batch. Drain on paper towels.