Heat oil and vinegar in a skillet over medium-high heat. Add chicken; cook until white on all sides, about 5 minutes. Transfer chicken to a plate.

Stir leeks into the skillet; cook for 1 minute. Pour in wine; simmer until reduced by half, about 2 minutes. Add half-and-half, Parmesan cheese, salt, and garlic pepper; simmer until flavors combine, about 2 minutes.

Return chicken to the skillet and stir until heated through and coated with sauce, about 3 minutes. Season with salt and pepper.