I am now running Leopard 10.5 with Windows Vista in Parallels 5540 Beta.
Have some errors during setup but manage to solve them and now running well on the fourth installation.
Vista is a bit laggy compare to my previous Windows XP in Parallels.
I realise Leopard 10.5 manages swap files much better than Tiger 10.4. Running Parallels in Tiger will use 1 to 2GB of swap files. With Leopard, I hardly use up to 64MB of swap files when Parallels was running.