I wish I had an answer for you... with Vista it could be anything.

Is UAC on or off?
On the next line after the addprinterconnection, add:
? @serror
What is the error?
Have you installed the Vista Drivers on the Print Server?
There might be a GPO that needs adjusting.

I don't have an environment where I can help you test. It was just easier to remove Vista and go back to XP.