I put my rabbit cages over my worm bin (permaculture style), but all the worms died! I'm sure it's because of the urine. Does anyone have experience with this? What can I do to make this system work?

Something which may work a bit better would be to till up the ground first, then put the worms down. The worms are going to need to be able to go into the ground at some point, and doing so if/when the soil is too hard becomes impossible.

Watering the ground from time to time may help, too, as will putting down some sort of surface covering (i.e. shavings, leaf mulch, grass mulch) to prevent the ground from hardening and drying out as fast.

There is quite a bit of discussion of vermicomposting in the Other Ways to Profit from Rabbits forum. I'm moving this topic to that forum to keep the topic together.