Which VLAN value permits virtual machine guest tagging in vSphere 5 environment?
Page 14 and 15 from vsphere-esxi-vcenter-server-50-networking-guide.pdf
A VLAN ID, which restricts port group traffic to a logical Ethernet segment within the physical network, is optional. For a port group to reach port groups located on other VLANs, the VLAN ID must be set to 4095. If you use VLAN IDs, you must change the port group labels and VLAN IDs together so that the labels properly represent connectivity.
If you enter 0 or leave the option blank, the port group can see only untagged (non-VLAN) traffic. If you enter 4095, the port group can see traffic on any VLAN while leaving the VLAN tags intact.