Afaik, that VMX key can not be changed through the ExtraConfig property.
If I understand correctly, you want to remove a NIC that is present on the VM(s).
Do you have the NIC type or NIC label for the NIC that needs to be removed ?
If yes, it would be a lot simpler to use the Remove-NetworkAdapter cmdlet.
Something like this
Get-VM-NameMyVM|
Get-NetworkAdapter-Name"Network adapter 2"|
Remove-NetworkAdapter-Confirm:$false
If you want, you can first have a look at the NICs connected to that VM as follows
Get-VM-NameMyVM|Get-NetworkAdapter
If you want to add a NIC, you will have to use the New-NetworkAdapter cmdlet.
Something like this
Get-VM-NameMyVM|
New-NetworkAdapter-NetworkNameportgroupname-Typevmxnet3-Confirm:$false