Place 2 tablespoons of the soy sauce and 2 tablespoons of the rice vinegar in a large bowl and whisk to combine. Add the tofu and toss to coat. Set aside.
Next, make the sauce. Place the cornstarch in a bowl and whisk with 2 tablespoons of water. Pour in the broth, remaining 4 tablespoons of low-sodium soy sauce, 2 tablespoons rice vinegar, vegan oyster sauce, sesame oil, and agave. Whisk again to combine and set aside.
Heat 1 tablespoon of oil in a large sauté pan or wok over medium-high heat. Once the oil is hot, add the tofu and cook, stirring often, until light brown and slightly crisped around the edges, 10 to 15 minutes. Remove and set aside.
Heat the remaining tablespoon to the same pan over medium-high heat. Once it’s hot, add the diced mushrooms and cook, stirring occasionally, until they release their moisture and the liquid has cooked off, 5 to 7 minutes. Continue cooking for another 2 to 3 minutes, stirring occasionally, until the mushrooms are browned and slightly crisped around the edges.
Add the onion, garlic and ginger and cook, stirring often, until the onion is translucent and the garlic and ginger are fragrant, 2 to 3 minutes.
Add the zucchini and cook for another 3 to 5 minutes, until slightly tender.
Add the reserved tofu and pour in the marinade. Mix to coat in the sauce and bring the mixture to a simmer.
Reduce the heat to medium-low and continue to simmer until the sauce thickens, 3 to 5 minutes.
Serve immediately with cooked rice (optional) and garnish with toasted sesame seeds and thinly sliced green onion. Enjoy!