Permissions to post comments, but not write wiki articles.

I would like to give some usergroups permission to post article discussion comments, but not edit wiki articles themselves.

Is this possible in Vaultwiki?

Yes. Customize the forum permissions for the forum.
Can Post New Threads? No.
Can Reply to Own/Others Threads? Yes.
Can edit wiki articles? No.

That's great news. That means that I can import my existing Wiki discussion thread (normal threads) into Vaultwiki discussions,. I should be able to do this by just merging threads. Is this correct?

This will only work if the discussion started AFTER the article was created. This should be the case for most discussions, but on my (other) site sometimes we discussed articles before creating them, which meant we had to change some timestamps...

Yes, I figured the time issue would need to be considered on each import. That will work then.