Normal hardening of the hands is a part of vaulting, but if you are getting extreme callousing in just one or two places something might be wrong with your grip. Someone once asked Sam Snead where his hands were calloused and he held up his hands to show that they were smooth. The lesson was that because his grip on the golf club was secure and right for his swing there was no unusual pressure or friction on any part of his hand. I think this is true for the vault as well.
You can pick your nose and your friends, but not your friend's nose.