Cook the pasta in a large saucepan of salted boiling water according to the pack instructions until al dente. Drain, reserving at least 50 ml (2 fl oz) of the cooking water, and return to the pan. Stir in the cheeses, adding enough of the cooking water to make a creamy sauce, and season.
Whisk together the vinegar and oil, then toss together with the watercress in a bowl and season well.
Spoon the pasta into serving bowls and serve topped with the watercress salad.