There's also another option. Apple changed their license for OS X starting with Mountain Lion to allow running the OS in a VM. So if 10.9 ends up borking Vectorworks, one can use Fusion or Parallels to run 10.8 and Vectorworks. Both have an option to do so transparently, so Vectorworks will appear as a normal program.
It would be a bit more complicated and probably a bit slower, but it's good to know there's a backstop if 10.9 or any future OS update is a problem.
-Bruce