The Production Portgroup has no VLAN IDs associated.
The VLAN vDS portgroup is configured for straight VLAN with a ID of 20.
All three hosts are plugged into a Cisco SRW-2024 Gigabit Switch.
ESXI01 - VLAN vDS dvUplink (vmnic4) is plugged into the Cisco switch at port 21.
ESXI02 - VLAN vDS dvUplink (vmnic5) is plugged into the Cisco switch at port 15.
ESXI03 - VLAN vDS dvUplink (vmnic4) is plugged into the Cisco switch at port 5.
The Cisco switch is configured as follows:
VLAN ID 20 was created as Static
Ports 5, 15 and 21 are assigned to VLAN ID 20.
I am inclined to believe that you are correct about it being a switch issue. The switch does indeed support 802.1q tagging, and I have configured the ports in several different ways (trunk mode, access mode etc) but have not been able to change the behavior of the issue. I may try to aquire a Cisco Catalyst 2960 from a friend to test with. Any other ideas that you may have are appreciated!
Greg