Just a little update from my side.
While writing the first blog entry, it looks like the list feature is working for blog entries, not for custom pages, though.

The fact that tags can't be changed after saving a blog (or draft) for the first time is confirmed.
I could imaging it has something to do with the tagging system in general: https://www.vbulletin.com/docs/onepage/#item62 - maybe those options could be checked?