Cisco VPN bandwidth usage by user

I would like to see the ability to report bandwidth usage by the user account when connected via VPN.  When a user connects via VPN and browses the internet this traffic is not reported as being sourced from the VPN IP that the user was assigned, but by the firewalls outside interface IP.  This makes it impossible to know how much internet bandwidth this user used while connected to the VPN.  The username and source/destination IPs are reported in the firewalls syslog, so a way to correlate this information into a report would be very handy.

Thanks,

Abel