vBulletin doesn't use cascading for external feeds. In other words, a feed that specifies a given forum won't also show items from its child forums. So selecting the feed for the wiki category will only show threads/articles in that immediate forum category (and since its a category, it can't contain any). This is standard vBulletin behavior.
However, I believe the wiki home page should have its own feed that DOES cascade and include child forums, so you can have a feed of the entire wiki. I am looking into this now.