You can have report on VPN usage by configuring accounting feature in your ASA. Accounting tracks traffic that passes through the security appliance, enabling you to have a record of user activity. If you enable authentication for that traffic, you can account for traffic per user. If you do not authenticate the traffic, you can account for traffic per IP address. Accounting information includes when sessions start and stop, username, the number of bytes that pass through the security appliance for the session, the service used, and the duration of each session.
For configuring the accounting in ASA refer the below URL:
http://www.cisco.com/en/US/docs/security/asa/asa82/configuration/guide/access_fwaaa.html#wp1043741