Rub pork tenderloin slices with 1 tsp. salt and 1/2 tsp. pepper. Cook pork in hot oil in a large skillet over medium-high heat 4 to 5 minutes on each side or to desired degree of doneness. Transfer pork to a serving platter, and keep warm.
Add garlic to skillet; sauté 30 seconds, stirring constantly. Add mushrooms and remaining 1/4 tsp. salt and 1/4 tsp. pepper to skillet; sauté 3 minutes. Stir in vinegar; cook, stirring frequently, 3 to 5 minutes or until mushrooms are tender. Spoon mushroom mixture over pork, and serve immediately.