Try to look into using your router as an access point. Have all the computers just plugged in normally to the router, and have the vista laptop connect to either (preferably the access point if it allows you to do so.)
On the kubuntu machine, you may want to use xubuntu if you haven't set it up yet. Either way, look into samba. That's what you'll want to use to do sharing with windows. I don't really know what you mean by using that instead of your flash drive. If all you want that machine to do is have files on it, google FreeNAS, that should do that task, well, easier.