It is possible to configure your FortiClient VPN to only use a specific adapter, but the exact steps will depend on the version of FortiClient you are using and the operating system you are running.
In general, you can follow these steps to configure your VPN to use a specific adapter:
Open the FortiClient VPN application.
Click on the "Network" tab, then select the "Custom" option.
In the "Adapter Binding" section, select the adapter you want to use for your VPN connection from the dropdown menu.
Click on the "Save" button to save your changes.
After following these steps, your VPN should only use the selected adapter for the VPN connection, and the other adapter should continue to work as normal.
Note that this will only work if you are using FortiClient version 6.0 or later, and if your operating system supports the adapter binding feature. If you are using an older version of FortiClient, or if your operating system does not support adapter binding, you may need to use a different method to configure your VPN to use a specific adapter. You can refer to the FortiClient documentation or contact your network administrator for more information.