Thanks Kid, I know that a layer 3 switch would be required to route between VLANS, which is why I'm asking about an overlap. I've downloaded the Cisco Network Assistant and am poking around with it, think I'm on track.
Right now I'm on site, and have a slow day, so this is more of an experiment/educational exercise. Currently I have a single distributed control system with two physically isolated networks, one for IO, and one for SCADA. With 2 1756-EN2T cards in my main chassis for each network. Late in the game I discovered I needed to code MSG instructions to exchange PLC data between a couple of smaller skid system. These systems are simple L35 controllers with flex IO over Ethernet for IO, and a single PanelView+.
Originally I wanted to keep all IO traffic isolated, however with these systems the PV+ and IO are on the same single network. I figured it would be wise to create some VLANS to isolate this skid systems.
VLAN1 - 1756-EN2T(SCADA), SCADA Terminals/Servers
VLAN2 - 1756-EN2T(SCADA), Skid 1
VLAN3 - 1756-EN2T(SCADA), Skid 2