Tags: user must enter at least one tag

[Lynne]

There is no default vbulletin function you may toggle to make users have to enter a tag.

You should probably look at the fetch_valid_tags function as that is the function used to check the tags and spit out errors. I don't think there is a hook in there, so you may have to actually modify the code. You can look in the zoints thread tagging modification to see how he did it and just use that code.