I don't have any first hand experience with a mis-tensioned belt, and can't give you an answer on the vibration. But I don't think the tension being off would affect how it runs, as long as the 3 pullys stay in synch on the belt. And that's what I think mis-tensioning would lead to. Sooner or later something bad is going to happen: either one of the pulleys jumps off the belt if it's too loose; or the belt frays and eventually breaks if the tension is too tight. But again, you may have gotten lucky and tensioned it ok - the pointers will give you the answer to that question.