And, regarding /vendor partition, because I also had no vendor appear after flashing 5.1.4 firmware, what's the consensus now: go back to full OOS for once, because the bootloader needs updating?
I don't think there's a consensus yet, but flashing OOS 5.1.4 was the only thing that worked for me eventually. Since OOS 5.1.5 is already requiring a working vendor partition (updater-script is checking for it), the "vendor partition creation magic" should happen somewhere inside of the official OOS 5.1.4 Build. (Be it at installation or at boot time.) So, what worked for me:
Edit (08/30/18): Better head over to
Post #1704 by
@Macusercom for a faster and more secure way to do this - especially if your device is encrypted.
Edit (08/31/18): Solved in Post
#1721 by
@codeworkx
1. Backed up my LOS 15.1 installation with official TWRP 3.2.3-1
2. Flashed official OOS 5.1.4 (=full ROM), also with TWRP 3.2.3-1
3. Let OOS 5.1.4 boot (in case vendor partition is being created during boot or while OOS 5.1.4 is running for the first time)
4. Reboot to recovery (which should now be overwritten by stock recovery due to step 3)
5. Install official OOS 5.1.5 (if this works, vendor partition is in place and working, because the updater-script of 5.1.5 cancels the installation, if it's missing.)
5a) If data partition has been encrypted before step 1, it might be necessary to flash 5.1.5 via OTG sideload (because stock recovery can't decrypt data encrypted by LOS 15.1)
5b) If data partition has been encrypted before step 1, OOS 5.1.4/5.1.5 might get stuck on first time setup - that's no problem for our cause, just ignore it and reboot
6. Let OOS 5.1.5 boot, just for the fun of it and to confirm everything went OK
7. Reboot to fastboot mode ("fastboot flash recovery twrp.img")
8. Reboot to TWRP and restore backup from step one
It's not the simplest method, but it worked for me. On encrypted devices one or two extra steps might be necessary, especially a full external backup of sdcard might be wise, in case you end up having to do a full wipe or TWRP can't decrypt data anymore.