Q: Is there a way to view all (get a list of) current Tags in VersionOne? And then also display which tags are PBI, Portfolio Item, etc.?
A: You can see all the created Tags in the Admin->Tags page, but there is not a way to see which ones are used specifically for on a certain asset type. The tags are expected to be global in usage, meaning you can use the same tags on backlog items, Portfolio items, defects, etc....
There is a way to see what workitems are tagged and what they are tagged with through the API.
This will show all your workitems in XML format and their tags, along with what the asset type is, so you can see what tags are used with what asset type. It will not show items that do not have tags.
You can then use an XML reader to parse that into a report format.