Bring the water to boil in a small saucepan or heat it for about 2 minutes in the microwave. Pour the hot water over the sun-dried tomato halves. Allow them to soak until tender (15-60 minutes). Reserve 1/4 cup/60ml of the soaking water, then drain the tomatoes.
Bring a large pot of salted water to a boil. Cook the pasta according to package instructions.
Place the soaked tomato halves, cashews, nutritional yeast, salt, basil, oil, and 2 tablespoons of the reserved tomato soaking water in the bowl of a food processor fitted with the S blade. Process for a minute. Stop and scrape down the sides of the bowl with a spatula. Continue processing for another 2 minutes, or until you have a thick, evenly mixed pesto sauce. If the sauce seems too thick to you, add the remaining 2 tablespoons of tomato soaking water. Add red wine vinegar and freshly ground black pepper to taste.
When the pasta is ready, reserve 1 cup of the salted pasta water. Drain the pasta. Add the pasta back to the pot and add the pesto. Mix well, adding a few splashes of hot pasta water as you go to help make everything smooth and saucey. It'll seem as though there's a lot of sauce at first, but as you mix, the sauce will be absorbed and find its way into the crevices of the pasta.
Top with vegan parmesan, if desired. Serve or store.