the vacuum hose applies vacuum to a diaphragm-like device known as a vacuum advance canister. Its purpose is to advance ignition timing under certain driving conditions, providing more power and responsiveness. If you take the hose off and plug the ends it hooked to, the car may become a bit sluggish. If you don't plug the holes, the ensueing vacuum leak will cause poor starting, rougher-than-normal idling and poor fuel economy. This sounds like a bad idea for a street-driven Volvo.