TL;DR: SteamVR relies on older C Redistributable packages. If you've removed them or they've been corrupted, you may need to reinstall them. I'm not sure which old one it is for SteamVR specifically, but there are downloads to get all of them since 2005 (I used https://www.techpowerup.com/download/visual-c-redistributable-runtime-package-all-in-one/). For me this instantly fixed a "Headset Not Detected 108" / "Please Plug In Headset" / "DriverManager lighthouse load error 126" error I had. Nothing else helped.

My Vive pro had been working perfectly fine until recently, when it suddenly wasn't being detected. I tried to search all the different ways that it could potentially be fixed, but nothing I found managed to work. Unplulgging/Replugging, power cycling the box and the headset, changing usb ports, verifying integrity of steamvr, reinstalling steamvr, reinstalling steam, uninstalling the usb drivers, restarting after everything, absolutely nothing I found and tried helped.

Finally I got on to looking through the logs in the developer web console, and found an error referring to "DriverManager lighthouse load error 126" in a particular DLL file. There wasn't much information around that, and I had the DLL file just fine, but I had recently just fixed a problem of Firefox not starting due to a 'side-by-side' configuration error by removing all of my C Redistributable packages and reinstalling the latest one for 2015-2019. I hadn't noticed steam trying to install any such packages when I reinstalled steamvr and steam, and originally thought that meant it simply needed the latest, but decided to check if it was somehow thinking one of the older ones I had removed and not reinstalled was still hanging around.

I went and found a download for all of the C Redistributable packages (this is the one I found: https://www.techpowerup.com/download/visual-c-redistributable-runtime-package-all-in-one/) and installed all of them through the batch script. On restarting, SteamVR instantly worked and picked up the headset.

In the vain hope of trying to help someone in the future from having the pain of fruitlessly trying all the standard responses of replugging and reinstalling steam, I figured I would post up my experience and resolution. Perhaps it will help someone later.