I have not seen any publicly released documentation on vmnet-cfgcli however executing it without any argument shows its usage.
Then, as an example command usage:
WKZs-Mac:~ WKZ$ sudo /Applications/VMware\ Fusion.app/Contents/Library/vmnet-cfgcli getdefaultbridge
Library Loaded
vmnetcfg: vnlgetdefaultbridge - Default bridged vnet is: 0