Heat the oil and butter in a large deep frying pan. Add the onions and cook for 5 minutes, until golden. Meanwhile, finely dice or slice the remaining vegetables. Add all the vegetables to the onion and cook for 8-10 minutes on a slow heat stirring frequently. Season well with salt and freshly ground black pepper. Stir in the mascarpone cheese until melted.
Meanwhile bring a large pan of salted water to the boil. Add the pasta to the boiling water once the vegetables are ready. Stir the pasta well with a large fork to separate the strands of vermacelli. Cook for 3 minutes until al dente. Drain the pasta, reserving 150ml (1/4 pint) cooking water; add the vegetable stock cube to the water and stir to dissolve. Add the pasta to the vegetables and toss well, using some of the reserved water to loosen the pasta. Stir in the mozzarella allowing it to melt. Adjust seasoning if necessary.
Divide the pasta between large bowls and drizzle with the extra virgin olive oil and balsamic vinegar to serve.