It's always more complicated than one expects
The game has a limited ability to represent political situations, which is okay. But my feeling is that the Vassal status doesn't fit as well as it might--that being said, without knowing everything that Vassal status entails.
On the one side, other major powers cannot directly DOW Moldavia and Wallachia, and on the other side Wallachia and Moldavia automatically DOW any faction that DOW'ed the Ottomans. That just feels a bit clunky. Better would be that the note stating that Wallachia and Moldavia are Vassals of the Ottomans, also stated that the Ottomans, Moldavia, and Wallachia all share the same political status, that DOW can only be played on the Ottoman faction, which will cause the DOW'ing faction to be at war with all three. And then DOW'ing the Ottomans should put the issuing faction into war with all three at the same time.
Again, it's probably minor, but it just feel clunky, and more over the information is missing from the message stating that Moldavia and Wallachia are Vassals of the Ottomans.