In reality Vauxhall has only run as a brand for a number of years. The plants - wherever they are located - produce vehicles badged as both Opel and Vauxhall.
Losing the brand structure would be the most sensible thing. Elsmere Port would only be at risk if the bottom fell out of Astra production. Luton's car production went with a wimper in 2002, so I imagine that the only risk outside of Griffen House would be van production.