When the VIA client establish the connection to the Controller, Usually we will get the MAC address of 00:00:00:00:00:00 and we won't get the real MAC address of the clients. VIA clients will be placed in the user-table based on their username and Password.
Starting from 6.2.x and above, there is a way to check the MAC address of the VIA client.
When we issue the command "show user-table ip <inner-ip-address>" (inner-ip-address is client IP from VPN pool)
NOTE: Below attahed screen shot we can see the VIA Device ID highlighted in black which will be the MAC address of the Client devices.
In this way, we can verify the MAC address of the Clients.