If one server has virtual hosts on it, with individual access_log files within each host, if i added up the data transfer from each virtual host, would it give me a rough guide to the bandwidth i was using on that server ?

Perhaps. It would give you a pretty close estimate of how much transfer your websites are doing. But the total transfer for the server also includes email (SMTP, POP3), FTP, etc. If you rarely use any other services on the server, you should get a fairly accurate number from simply the webstats.