It's either the Windows firewall on the Vista system - or - Vista and it's security peculiarities. Unfortunately, not being a Vista user
I can't help with either.
I sort of knew Vista would be the issue.
However, got a bit further now. Vista will now see the \\Nas\lp and allow me to add the printer, so far so good.
I have disabled bi-directional but when I try to print it comes back with an error.
I have also run secpol.msc and enabled 'send LM & NTLM - use NTLMv2 session security if negotiated' but this has not made any difference.
So, I am getting there slowly but surely.