If you haven't installed the guest additions in the Vista guest, then
VirtualBox will be capturing the keyboard and mouse. To get control
back into your Ubuntu host, you need to press the 'host key' which, by
default, is the right hand ctrl key (*specifically* the right hand one -
the left hand one won't do it) - this can be changed from the Sun
VirtualBox control window by going to File > Preferences > Input but
not, so far as I can tell, from within the guest OS.

I know that VB can be a bit of a pig at first, but it is worth
persevering with - after much frustration and self-inflicted hair loss,
I now have several virtual machines running (including XP, Ubuntu and
numerous other *nixes as guests on both Ubuntu and Vista hosts) and I
wouldn't be without it.

Hope that helps, a bit, at least,