Hi Uwe
are you looking for this in ESXCLI:
esxcli network vswitch standard portgroup list
this lists some vlans
in Vmware, simply , right click on exisiting CVM(as you are doing expansion), go to settings,in front of Netwroking, you will see it is attached to which port group (take note of the name).
now, on ESXI , go to Networking section, Portgroups, you will see the port group name and the corresponding VLAN ID for it.
for CLI , you can run:
#esxcfg-vswitch -l
It will show you the port groups and corresponding VLAN ID
if you are doing foundation with FLAT switch, I suggest you do not put the VLAN ID now, you can tag the VLAN ID later when the foundation is over.