We've been doing this for years to fulfill a requirement by the HR Department. Our MX is sending logs to a Syslog server. The Syslogs contain unique wording for when a client is connected and disconnected from the VPN:
logtime message
2020-06-21 14:43:26 1592765006.928610814 MY_MX100 events client_vpn_connect user id 'employeeA' local ip 192.168.10.8 connected from 70.187.200.74
2020-06-21 15:16:16 1592766976.058749887 MY_MX100 events client_vpn_disconnect user id 'employeeA' local ip 192.168.10.8 connected from 70.187.200.74
If you can write a script or program to parse that out for you, you can get the username, the local IP, the remote IP and the timestamp. We put all these VPN events into a SQL table to make reporting simple.