I've also tried to run the gui client under mono. The mono project has come a long way; it now has an OSX build and includes the WinForms stuff. However, the gui client gets stuck on the registry since the mono project doesn't include support for it.
When I search for Vault and OSX I keep getting my hopes up when I find the 1993 announcement by Eric that you are working with the Mono project to bring Vault cross platform.
At this point I've installed Subversion and imported my projects to it. It's a poor substitute for Vault (due to the really nice Vault client), but it does work. If I can get some word on the Mono situation I'll stick with Vault (which I recommend to everyone I can

Thanks