A vendor image mismatch has been detected. Typically this means image is out of date. Please ensure your vendor image matches OPM7.181205.00
What this rather scary message essentially tells you is that LineageOS update was built on newer version of your phone's image than the one you have installed. For me that meant a visit to Google's firmware page and download of factory image for Nexus 5x.
Assuming one wants to keep LineageOS and not downgrade to the official firmware, we have to go into downloaded zip file and find another zip file within. It's in that inner file one can find
vendor.img in the same folder you already have platform tools in (I just assumed you have one) and get the phone into
fastboot. I personally love Advanced restart functionality withing Developer menu and I simply reboot phone by pressing power key a second or two and selecting Bootloader. However, one can also do it from platform tools command prompt:
List of devices attached
adb reboot bootloader
If there is no device listed at all, make sure you have USB Debugging turned on in the Developer menu.
Once your phone gets into bootloader, we simply need to upload
vendor.img followed by a final reboot:
fastboot flash vendor vendor.img
Sending 'vendor' (190332 KB) OKAY [ 4.299s]
Writing 'vendor' OKAY [ 3.098s]
Finished. Total time: 7.431s
Finished. Total time: 0.009s
Pesky message should be gone until a next vendor image comes along.