To use the Portgroup, you could do something like this
$vmNic = (Get-NetworkAdapter -VM $vm | where{$_.NetworkName -eq <your-portgroup-name>}).ExtensionData
To use the vNIC label, you can use the same logic as in https://communities.vmware.com/t5/VMware-PowerCLI-Discussions/Remove-Port-Mirroring/td-p/2250195/jum...
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