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"