According to some blogpost, the following works:
ethernet1.connectionType = "pvn"
[update] You also want to include the following:
ethernet1.pvnID = "52 dd bc d5 36 19 1b 6b-0f f1 fb 1c 4c ac 44 f7"
Then, open the VM and create/select a LAN Segment from the GUI.

From the vmware knowledge base:

ethernet1.connectionType = “custom”
ethernet1.vnet = “vmnet2”