You dont have to change the artnet input ports in catalyst - they are virtual... (and fixed)
Just change the patch in the catalyst control window. (see pic below)
Select the artnet output port on the Vista to match (sounds like you already have...)
Ensure that the Mac IP address is 2.0.0.x and subnet is 255.0.0.0
(See catalyst manual...)
http://sites.google.com/a/samsc.com/catalyst/dmx-set-up
Simon