Perhaps you turned on the 'tmux-MacOSX-pasteboard' program?
I also had this issue, after I upgraded from Snow Leopard(10.6) to Mountain Lion(10.8).
Like you, I had nearly one panic a day. The kernel panic log showed: "negative open count"
Upgrading/Compiling MacVim did not solve the issue. I was using 7.3 before upgrading the OS.
I recalled that I was using the 'tmux-MacOSX-pasteboard' program before upgrading to Mountain Lion.
https://github.com/ChrisJohnsen/tmux-MacOSX-pasteboard
I wasn't sure it had anything to do with it, but at least it was something that was interacting with the lower level and Vim, so I thought it was worth a try.
I decided to turn off that program, and see how well it goes along.
That was early November... and I haven't met any crashes since then.