Check the front tire/wheel. The tire could be unbalanced, out of round, or just plain defective. It happens occasionally even with today's tire technology. The front wheel may be wrong too. Loose spokes, out of round, or even improperly torqued axle bolts. Have an independent M/C mechanic check it out if the dealer won't listen to you.
Isn't that M50 a single? I bet that the vibration is pretty much built in.
But as everyone said, if you can ride it with reasonable comfort up to 75, but the windscreen is causing buffeting, then you need to figure out what to do with the screen. I don't necessarily agree with mscuddy that you should ditch it, but I think you need to figure out how to adjust it so that you can ride comfortably.
Here's what you need to do: First, find out at actually is going on. Can you see the windscreen juddering when you ride? Can you feel any vibration or movement in your hands from the handlebars -- in other words, is your front wheel being pushed around when you have the windscreen on at higher speeds? If you can't see it moving or feel it pushing the wheel around, then what you are feeling is likely to be air turbulence that's bouncing your helmet around. And that's easy to fix (relatively.)
Turbulence is when the air "breaks" and rolls around. If you suspect that it's turbulence, try moving your head down closer to the tank, and see what happens. Does the buffeting and "vibration" go away? If so, then your problem is that your head is right at the turbulent line between the fast clean air flow above the screen and the still air below it. You can fix the problem by moving the screen around, or cutting it down, so that your head is either in the clean air flow or in the still air pocket.
I don't know what kind of screen you have but many screens allow you to adjust the angle and height of the screen. You can play around with those adjustments to see if you can get your head out of the turbulence. Also, if all else fails, take a hacksaw to it and cut it down.
I hope this helps! It's hard to diagnose a problem from a distance.
I've gotten to ride a 2006 C50 (similar to an M50) for a weekend and everything from 5-90 mph. It buzzes like most twins at higher speeds, but nothing I found out of place (i.e., it's good to hop off every 45 minutes or so to get the feeling back in hands and feet). It had a big old national windshield and ran just fine.
Assuming it's not just that twin vibration that you mistakenly thought would be smoother, it sounds like a wheel or frame issue to me.
Just a thought...there's a difference between wind buffeting and actual motorcycle vibration. If you are sure that the bike itself is shaking, and it always has done, and the windshield makes it worse, then check that the steering head bearings are properly adjusted. Get someone who knows what they are doing to check them for you - and the wheel bearings, too. Basically, if you can get the bike on a lift or on blocks so that its front end is clear of the ground, and you can feel actual movement if you grab the front wheel and push and pull the whole assembly backwards and forwards, then that's your problem. Fitting a windshield will provide a larger reciprocating mass on the other side of the fulcrum, thus making the problem worse. To buy a brand new bike with incorrectly adjusted bearings is not at all unknown...
Ditto on the head bearings. Does the bike tend to 'flop' into turns at lower speeds? It's another loose head bearing indicator. Your's wouldn't be the first to be loose from the dealer.
Speaking of which, if a dealer EVER told me it was in my head, I'd write them up with the Suzuki regional office and find another dealer! Not even a look see or test ride? So much for customer service.
Another thing to check is if the windshield is mounted correctly. If it's off-center the wind will push the shield, and therefore bars, to one side. The shakes could be you trying to compensate for this push.
+1. Every single new bike I've had (all 2 of them) the steering head bearings were not properly adjusted. I don't know why dealers have such trouble with this, but I'll bet it's the source of your problem.