As for CAL used in VDI, you could check below FAQ to understand better.
Licensing the Windows Desktop for VDI Environments
Besides, quoted explanation from Dashrender "To use VDI (Virtual Desktop Interface) you need and OS license (either a workstation license with Software Assurance or VDA or Intune [combined with a desktop OS license]) and a connection method license, such as RDS or VMWare View or XenDesktop." in a similar discussion.
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Last but not least, you could reach VLSC support for a call and consult the questions of CAL.
Hope this helps and please help to accept as Answer if the response is useful.