I have not used virtual box on Windows, so I can’t give you an answer for that, but in general, I prefer to use network shares instead of the “shared folders” features from virtualization softwares. I have had stability problems when using such features on ms virtual pc, and on vitual box on ubuntu.
If you want to do that, you will need to install samba on the Kubuntu guest, and configure the shared folder. Then you can access that folder from vista as if it was on a separate physical machine on the network (i.e. \\Kubuntu_machine_IP_Address\your_shared_folder )