Place cornstarch on a small plate. Heat oil in a heavy skillet over medium-high heat. Pick up one piece of tofu at a time, let excess marinade drip off (reserving marinade). Dredge in the cornstarch to coat on all sides, shaking off excess. When oil is shimmering hot, fry tofu in single layer, flipping only when it releases easily from the pan. Fry on all sides, then transfer to a plate. Add vegetables to same pan and cook, stirring frequently, until crisp-tender. Return tofu to pan and along with remaining marinade and toss to coat. Cook a minute or two until sauce thickens.