> I use this mask to rename files in a directory. :%6:_:01:.*
> everything gets renamed using the first directory name.

this is incorrect, not 1st dir name, dir name takes what %6 takes

you can try this way, it will work

1. put the ztree cursor over a directory that has many subdirectories then b
2. alt s
3. p
4. enter
5. tag files
6. control r
7. :%6:_:01:.*