Are the rpm's dropping with the ac on causing the vibration?

 

Vibration might be the RPM dropping too low, like Poorman says. In fact, might be the most likely reason.

Or it might be something with the drive belt. Watch the compressor and drive belts as the AC cycles on & off. Any pulleys that are wobbling under the increased load of the AC?

Maybe it's normal? Just the increased load of the AC compressor requires the engine to work harder. You can hear that, and feel it in a subtle way. If you've only owned bigger engines, maybe it's just a comparison to what you're accustomed to? Not to be a jerk, but I'm trying not to take anything for granted either.