Hi SvK,
VI 1.12 does not work under Leopard in its current version, since the final build of the OS was only revealed upon release and all our efforts then went into the development of Vienna Ensemble.
I would not recommend to install any freshly released OS on your work system anyway, since common experience shows that it usually takes a couple of OS updates to allow the integration of all the components of your system in the new Operating System.
I usually create a dedicated partition for the new OS to experiment with it until I feel that it runs smoothly with everything I need from it and only then make it my main system.
I don´t say "never change a running system", but I learned the hard way to be very careful with major updates, especially of the OS.
We are working on it, please bear with us...
Best, Marnix