No functional changes to the Pledge Vassals skill would actually be made.
The table might look something like this.

This table would allow the Dark Oppressor to see a list of all current vassals, and how much DP they've been earning for the Dark Oppressor. It would function as a performance leaderboard of sorts, allowing for the Dark Oppressor to encourage some friendly...competition amongst its vassals if they so chose.
I think this addition would add a significant amount of flavor to the Machiavellian/ruling aspect of the Dark Oppressor class, while requiring no changes in skill balance/functionality.