The /vendor partition was first introduced on OnePlus 5 with OxygenOS 5.1.4. You will first need to flash that image on your phone so that the proper partitions are created. This post on xda-developers forums has download links to older firmware.
I would recommend reverting to stock recovery image (download link in the same post), then flashing OOS 5.1.4, ...
"Format Data" via the TWRP recovery -> Advanced Wipe -> Reboot -> Reinstalling LineageOS 16 seemed to do the trick, and the OS now loads.
(Interestingly, I still cannot identify the device via adb/fastboot, I wonder if the stock ROM will resolve that).
This flashable zip does nothing more than installing some OTA startup script which does modification in build.prop
assuming you have unlocked bootloader you can install flashable zip from custom recovery via fastboot/adb
fastboot boot twrp.img
adb push android_lowram_enabler.zip /tmp
adb shell twrp install /tmp/android_lowram_enabler....
Fix Device not showing up in Fastboot Mode
So it turns out Windows doesn't like the OnePlus USB drivers because they are not signed. So you have to bypass it by disabling driver signing enforcement.
install the OnePlus drivers that are included with your phone.
--- they should show up as CD Drive: OnePlus Drivers when you just connect your phone to your ...
I found a simple solution without requiring a third party app.
Just connect to the device using shell: adb shell
Then run: logcat | grep your.package.name
It will only show lines that contain that package name.