You can patch BOOT + VBMETA using the magisk app !
Yes, magisk app will patch VBMETA too !
Just follow this:
1. Download the full stock rom.
2. Extract these two files: boot.img and vbmeta.img.
3. COMPRESS those two files to TAR. (Attention: TAR and NOT tgz. You can use 7-Zip: https://7-zip.org )
4. Open the magisk app and click on "install to file".
5. Select the COMPRESSED TAR file and wait until it finish.
6. Open the folder "Downloads" and uncompress the file magisk_patched.tar to get the patched boot.img and the patched vbmeta.img.
7. Flash the patched boot.img and the patched vbmeta.img. (You need to flash patched vbmeta because vbmeta verifies if any partition was changed.)
Done, you got patched boot.img and patched vbmeta.img files.
Warning: magisk app will ignore if you try to patch ONLY the vbmeta.img, you need to patch vbmeta.img together boot.img compressed in only one file.
If you try to flash any custom partition, your device can bricks or gets in bootloop. This patch with disabled verity and disabled verification avoid it and need to be flashed once only. Once flashed, you don't need to flash vbmeta again.