You can try a step down converter like this.

ebay.com link: itm

Or you could use a adjustable motor controller like this.

https://www.amazon.com/gp/aw/d/B00HTZD9YI/ref=psdcmw_306530011_t1_B01M1HSYO7

I'm sure others here have some more ideas to slow the motor down. I am against the idea of cutting the weights due to it's a balanced assembly. You would have to weigh each weight on a postal scale to get it right or run the risk of burning up the motor bearings prematurely. Hope this gets some ideas in your head for you to play with.
-Mike

They are designed to shake,yes but the weights are equal so that the shaft does not tear apart the motor bearings.
These are just a smaller version of what we use on our dump trailer bodies at work.
Furthermore some shaker weights are offset, some are not. If you have the weights offset the motor spinning will give less vibration but it will last longer. We can usually get 4-5 yrs from the offset weight style motor at work.
The non offset in our clay bodies last about 2.5-3 yrs depending on how long the driver has it on to get the body empty.
So you don't think I'm full of shit, here is a video of a dump vibrator in action.
-Mike

You would be putting this in between the vibrator controller and the motor itself. I'm not sure how the mpu would know you slowed things down since it's after the controller.
-Mike