Thinly slice mushrooms. In a large nonstick skillet, heat oil over high heat. Add mushrooms; cook, tossing often, until browned, about 10 minutes.
Meanwhile, bring 3 cups salted water to a boil in a small saucepan; add asparagus. Cook until crisp-tender, 3 to 5 minutes; drain.
To mushrooms, add asparagus and scallions; season with salt and pepper. Cook, tossing, until scallions have wilted and pan is dry, about 2 minutes. Stir in vinegar; serve immediately.