Preheat the oven to 350°F (175°C).
Place the manicotti shells in a large bowl and cover with very hot water; soak about 30 minutes until softened, stirring gently halfway through.
While shells soak, drain any excess liquid from the vegan ricotta.
In a mixing bowl, combine the drained vegan ricotta, 1 cup of the shredded mozzarella, Italian seasoning, vegan Parmesan, chopped spinach, chopped basil, cornstarch, salt, and black pepper; stir until evenly blended.
Lightly spray a 9×13-inch baking dish with nonstick spray and spread half of the pasta sauce in an even layer on the bottom.
Transfer the cheese mixture to a piping bag or a zip-top bag with a snipped corner and pipe about 1/3 cup of filling into each softened manicotti shell, then arrange the filled shells seam-side down in the prepared dish on top of the sauce.
Pour the remaining pasta sauce over the filled shells and sprinkle the remaining 1/2 cup of shredded mozzarella evenly on top.
Cover the dish with aluminum foil and bake for 45 minutes.
Remove from the oven and let rest 10 minutes, then garnish with additional fresh basil if desired and serve.