In the case of OSXvnc, the executable is symlinked to the root of the app, so /Applications/OSXvnc.app/OSXvnc-server would launch the executable. You can pass arguments to this as well, so /Applications/OSXvnc.app/OSXvnc-server -rfbport 5901 would listen on port 5901 (useful if they have enabled the Apple vnc server on its default port).