Hi CSIEnvironments,
This rename operation actually destroys the vApp network, then creates a new one with similar settings and the new name. As you point out, having VM network adapters attached to the network, prevents destroying the network and thus prevents the rename. To work around this, use the Set-CINetworkAdapter cmdlet to detach all VM network adapters from the network. Then do the rename operation. Then attach the VM network adapters to the new vApp network (the one with the new name).
Of the top of my head, the code to detach the adapters should be something like:
$myVAppNetwork | Get-CINetworkAdapter | Set-CINetworkAdapter -vAppNetwork $null
Cheers,
Andrey