Heat vegetable oil in a large saucepan over medium heat. Stir in garlic and onion; cook and stir until onion has softened and turned translucent, about 5 minutes. Add shrimp, bok choy, broccoli, cauliflower, carrot, and green onion. Pour in water, cover, and cook until shrimp is no longer translucent in the center and vegetables are tender, about 15 minutes.

Dissolve cornstarch into fish sauce in a small bowl. Stir into the cap cai along with oyster sauce, sugar, and pepper; stir until thickened. Season with salt before serving.