Cook spaghetti in a large saucepan of boiling water following packet directions. Reserve 1 cup cooking water, then drain. Return spaghetti to saucepan, cover and set aside to keep warm.
Meanwhile, heat half the oil in a large non-stick frying pan over a medium-high heat. Add prawns. Cook for 2-3 minutes, turning occasionally, until changed in colour. Add garlic and chilli. Cook, stirring, for a further 1 minute or until light golden. Remove prawns and set aside.
Heat remaining oil in same pan. Add onion and capsicum. Cook, stirring for 2-3 minutes, until light golden.
Stir in pesto, tomatoes, vinegar and reserved pasta cooking water. Gently boil for 2 minutes. Return prawns to pan with snap peas, parsley and spaghetti. Toss over heat for 1-2 minutes, until combined and heated through.

To butterfly prawns, cut along the back of prawns with a small knife, without cutting right through. Press gently to flatten slightly.
Replace sugar snap peas with snow peas or asparagus, if preferred.