If you don't have any leftover pasta, cook 6 ounces of spaghetti until al dente, according to package instructions; drain, and rinse under cool water.
Heat 1 tablespoon oil in a large nonstick skillet over high heat; add tofu and 1 teaspoon salt. Cook,tossing occasionally, until golden, 8 to 10 minutes; transfer to a plate.
Heat remaining tablespoon oil in same skillet over medium-high heat. Add bell peppers, cabbage, garlic, pepper flakes, and 1/2 teaspoon salt;toss to coat. Cover; cook, tossing occasionally, until vegetables are crisp-tender, 8 to 10 minutes.
Meanwhile, make sauce: In a small bowl (or liquid measuring cup), stir together peanut butter and 1/4 cup hot water until smooth; stir in soy sauce and vinegar.
Add tofu, leftover spaghetti, and sauce to skillet; simmer until heated through and saucy, 1 to 2 minutes.
When cooked, extra-firm tofu holds its shape better than other types; drain well on paper towels before using.