Heat oil in a large skillet over medium-high heat. Add garlic and ginger and cook until fragrant but not browned, 30 seconds to 1 minute. Add broccoli and cook, stirring, until the broccoli is bright green, 2 minutes. Drizzle water and fish sauce over the broccoli; reduce heat to medium, cover and cook until the broccoli is just tender, about 3 minutes. Stir in vinegar just before serving.