In a small bowl, combine garlic, ginger, vinegar, soy sauce, and red-pepper flakes.
In a large skillet or wok, heat oil over medium-high. Add mushrooms and cook, stirring, just until tender, about 5 minutes. Stir in garlic mixture and cook until slightly reduced, about 2 minutes. Add shrimp and snow peas and cook, stirring, until shrimp are opaque throughout, 3 to 4 minutes. To serve, top with scallions.