Does it still vibrate after removing the blade and/or blade adapter? If so, its bent.
My wife hit something with our Honda powered walk-behind a few years ago and bent the crank. The mower was only a couple of years old and the shape of the deck seemed to mulch better than any other mower I had ever used so I kept it around figuring I would one day fix it.
Last winter, after pricing a new crank and a used Honda engine, I picked up one of the China specials at Harbor Freight for $80 (on sale for $100 and then an additional 20% coupon.) It bolted up and only required slight rerouting of cables (this engine is not a direct copy of the Honda as I have read on this sight) to get the mower back in action. It seems to run just as well as the Honda and holds a lot more gas... and it was way cheaper.