"Wait. Have you used these? Aren't they just a gimmick?"
I don't know about the bar inserts but there is a lot of that stuff used in sports gear, and it does work. I had some golf club shafts once with that in them and they had so little feel it was a bad thing. The are also used on archery gear, and work really well. A lot of this stuff is made by the same company and distributed under different names.
Another option would be a suspension stem. They don't kill vibe specifically, but they put a rubber link between you and the fork, and take the zip out of the bars to the extent that they aren't pounding to the same degree. Check out Softride PowerStem.