Take off the belt first ...
If the motor vibrates with no belt, check all the bolts attaching the motor to the mount. Then check both pulleys to make certain they are not loose or slipping. Then look at the belt for damage or bumps. Then take off the blade nut and rotate the blade 90 degrees and check for anything that would cause vibration. Rotate the arbor by hand and listen for scratching noises like dry bearings.
If all checks out and it still vibrates, call the shop and explain what you have and what you have done to inspect for causes. They will advise you what to do next.
The answer to your question will only be as detailed and specific as the question is detailed and specific. Good questions also include a sketch or a photo that illustrates your issue. (:< D)