Salt WILL kill most any vegatation and sometimes it will take a while for them to find it if you move it. They will still go to the old spot until the salt leaches out of the soil there.

Put it back in the same place. I think if you can watch you'll see the deer actually lick and eat the "salty" dirt more than they will the block. Those red ones also have trace minerals but they are still over 90% salt.

put a piece of flat rock in the middle of the wallow so when it rains the block won't be down in the puddle and if you put another flat rock on top of the salt block it will last a heck of a lot longer.