"It" probably read my post and decided to play hide-and-seek
Virtual Libraries and the Category sidebar have long confused me. I want to assume** that deleting a Tag from the Category sidebar within the context of a VL will remove it from the books in the current VL, and only delete the Tag itself if the resultant physical library usage count is zero, but it doesn't do that. But nor does it seem to do that in Manage Tags. The Tag is removed/changed from/in the physical library.
**I 'sort of' knew right click Rename/Delete on a category don't work as I thought they worked - i.e. limit the scope of the operation to the current VL, rather than apply it across the physical library. To me, it seems inconsistent to have left click operations operate within the subset of books defined by the current VL, whilst right click operations operate across all books in current physical library. C'est la vie... just another san fairy Anne moment
To remove the tag _silly from books in VL-A but leave it in books not in VL-A, I have to select VL-A, then select _silly in the Tags section of the Category sidebar, then select all the books displayed for search tags:"=_silly", and then do a bulk metadata edit to remove the _silly tag from those books only.
BR