Currently I monitor VPN by having a syslog server in place.
The configuration is a linux server Fedora with syslog-ng, Mysql, and php-syslogng-mysql.
Currently this model lets me know when contractor / consultants vpn in. THe syslog message from the pix tells me When howlong and how much.
Syslog-ng allows you to forward messages by filtered criteria (ASA/pix syslog seq numbers or textual syntax inthe message, to either email or MYSQL.
php-syslogng-mysql is a web guis that allows easy access to the log enteries in to MYsql.
isntallation is simply Yu the components, stopping syslog on linux and starting syslog-ng as it's replacement.