Just run the new printer wizard in Vista and let it search for
networked printers - it should pick up the networked printer.
Caveat is if this network printer is actually a USB one that's plugged
into the Ubuntu PC, install the printer driver as normal in Vista and
then when it asks for the USB cable plugging in you right click on the
USB icon in VBox and tick the printer ( this is the equivalent of
plugging in a USB device in VBox - un-ticking is the equivalent of
un-plugging ).

That will not do it if you do not remove the old driver first.

What driver ?? Mark hasn't even found out how to connect this printer
to Vista in VBox so how's it going to have a driver installed ? The
one in Ubuntu isn't going make a jot of difference to Vista

I learned about internet printer protocol and with a bit of
research, I found that I could define an ipp printer to Windows Vista.
Now, my Windows Vista Virtualbox guest of my Ubuntu host can print to a
laser printer connected to a different Ubuntu PC on my home network.
Thanks for offering help.

If anyone out there is looking to do the same thing, they should check:

howto.ccs.neu.edu/.../adding-an-ipp-printer-queue-to-windows-v\
ista/

He was talking about vista on another computer when I answered his
question on not VBox

I tried what you suggest but Windows can't find
any printer.

The printer is not USB attached to the VirtualBox host, but to another
Ubuntu system which shares the printer with the VirtualBox host and
which is active on my home LAN.

I have no problem printing from the Ubuntu host system, but the Vista
guest does not seem to be able to find any printers.

I also don't see the printer listed as a USB device in the device
pull-down available in the guest "window"

I am not sure if this fits in or not. On my systems here, I have a
Windows XP system with the HP printer attached via USB. I can print
from the XP host, a Ubuntu wired desktop, a Ubuntu wifi laptop and
another laptop running XP and wifi. All can print to the one printer.
I order to get this to work I had to make sure all were setup for
the HOME group of the XP system that has the printer attached. Other
wise I couldn't see the print server.
I can't remember just how I got the Linux systems to be in group
HOME but the XP ones you do from the device manager.

Vista needs a Vista driver to work from its side. You get that by going
to HP.com support and putting the printer model number in the box and
clicking the for Vista drop down..

Thanks for the suggestion. I solved the problem a couple of days ago
after learning about Internet Printer Protocol. I have installed the
remote printer as a networked printer on the Vista VBox guest and it is
working fine now. I can print to it from both my Ubuntu host and the
Vista guest across my home LAN.