In response to the shield grounding problem associated with the ring heater in-vacuum cabling, I cut the connection to the in-air D-sub backshell at the end of the cable attaching to the ring heater driver.  This cut effectively creates two portions of the system cable: an in-air shield (grounded at the driver), and an in-vacuum shield (grounded up next to the ring heater itself).  

Using this fix, there is no need to re-engineer the in-vacuum shield termination for the ring heater.