OK - so I have isolated what I believe is the conflict.
The Outdated VC Plugin error only occurs for me when Krisp’s Audio Driver is in /Library/Audio/Plug-Ins/HAL/
I cycled through each of my audio drivers (eg. Zoom, Loopback/ACE etc) and ML 4.9.1 worked fine with each of them.
Only issue for me was when Krisp was there.
I really do need to use Krisp for my setup (its an incredible piece of software!) but hopefully this will help you figure out what’s going on.
Many thanks to Oliver.
Update - if I move the Krisp driver out of /HAL/ the Krisp mic still works and so does ML 4.9.1. However if I restart the machine then Krisp stops working. So one workaround for the time being seems to be open with Krisp driver in HAL then move it out and then open ML 4.9.1.