Of course, when making this assessment you are assuming the current MediaWiki software. Unfortunately, the current software is inefficient at undoing large amounts of activity. For example, the User Contributions page should have an "undo all of this user's contributions" button, especially if it isn't many. At the same time, this undoing of all contributions should also be undoable with a single command. There should also be a selective feature, e.g. undo only all image uploads or undo only the article edits that involve adding an external link. All of these are non-trivial to code, which is why no-one has done it yet, but it would make cleaning up after vandalism much more efficient, even if the vandal is himself an admin.
Trying to code automated fixes to admin vandalism can have only limited effectiveness. A skilled vandal could easily find some form of vandalism that you haven't coded a way to undo yet. Or, better yet, use your "undo all contributions" button as a form of vandalism in itself. (If you restrict access to that button, then you've just recreated all the problems we have now, just with different names.)