Anyone have experience using vermiculite as a soil amendment for lawn building or in breaking up hard soil around yard trees?

Thanks

It's OK but there are more effective and less expensive choices - like lava sand, granite, expanded shale, DE, etc.

Hard soil is "caused" by the lack of healthy beneficial microbes. They have died. You can recover them by putting several inches of mulch under the trees and watering. The microbes need a continual amount of moisture rather than teeter-tottering from fully dry to wet. Mulch will solve that problem for you. Reapply mulch every year as it decomposes.

Vermiculite will not do anything for you if you are going to let the soil surface dry out. Mulch is what you want.