First check via Firewall/Monitor/Top 50 if the user is in the USER list and click on the User, then you can see the infos filtered for that user.
If the user is not in the Top 50 (select the proper Time Range (last hour....) then you may find some Info in statistics which per default goes back to 30 days:
Traffic Statistic Info is also under Statistics: Goto Server and select your Server, then your VPN-Service and then select Byte(Dst-Src).
Then select the Date in the left hand site and click on "Show".
Do you see any VPN Usernames as source ?
If not, look under VPN/Status via right-Click on the Username/Last Access what IP the User got in the VPN Clietn Subnet (Access Granted@.......)
And then look at above Statistics if you find the Source IP of the VPN-Client.