If you can get to a command prompt you can use someting like "ifconfig -v ppp0" before disconnection
and redirect result to a log file. Pppd has up and down scripts that can automate the process. I never
managed to get Vodafone software working on Ubuntu and use a manual connect/disconnect so adding
logging to this is trivial. It should also be possible to keep the log on the dongle but I've never tried on
mine although I did check it mounts as a drive ok.
Windows version of vodafone software seems less than useful for monitoring usage so I'd not bother
about it not working on Linux.
You can try to register for online billing and download a csv of usage albeit days behind and most
unfriendly format.
David