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 ).