I don't think I'm missing anything obvious but in the forum permissions there doesn't seem to be the ability to prevent users from replying to another members thread, as we currently have our VB installation setup on some of our forums.
Please tell me I'm wrong or there's a work-around...?
You are correct. There is no permission for that in XenForo. There is a permission for viewing others' threads, but not for replying to others' threads.
This is possible with an addon. You can post a request: