0-200 vibration
The 0-200 is a great little engine and should run as smooth as silk. On a good one you could place a 50p bit on the engine on its edge and it wouldn't fall over, when the engine is at 1500-1800 RPM. If the engine is vibrating as you describe, you definitely have an issue. There has been some good obvious stuff posted here, but one issue has not been suggested, so I thought I might.
The 150 engine has two impulse magnetos, the two magnetos should make an audible "Click" at approximately Top Dead Centre. The clicks should occur at the same time or very close together and be consistent with every turn of the propeller. The clicks are caused by the "Impulse Coupling" which retards the ignition to about TDC for starting; check for this.
It is not unusual for the spring in an impulse coupling to break causing the coupling to allow the timing in that magneto to "Drift". It is not unusual for the coupling to work even with a broken spring, since as the spring is wound-up it will trap it windings and will allow the coupling to work. However, when the coupling is unwound in its normal position, the break in the spring will allow the coupling to drift and as the coupling is on the same shaft as the timing "points", the timing will drift back and forth giving rise to vibration that is not in any way "Dynamic", in that it will not conform to a standard frequency but be very erratic.
If your vibration is not "Dynamic" and will come and go at different frequencies at different RPMs, then the cause may well be in the magnetos. Remove the magnetos and check them in accordance with the 500Hr inspection procedures. In particular, disassemble the impulse coupling and check the spring is intact. Also check the magneto rotor bearing at the rotor cam end. There should be no side-to-side "play" (movement) that you can feel. Any side movement on the rotor shaft will cause erratic points opening and closing which in turn will at best vary your ignition timing erratically, and at worst cause the magneto not to function intermittently.
Before "condemning" the engine it is imperative you check the easy stuff first. Further, I agree with the post regarding propeller position. Despite the fact the propeller can be fitted at any position, from experience and for a vibration issue, the propeller should stop at about 22minutes past 10 (10:22Hrs.) on an engine with good cylinder compressions.
Hope this helps,
Rgs. Bob.