To some degree. But the conditions under which alliances are formed and break and vassalage is formed and cease are different. And it can be relevant in different cases.Not when it comes to defensive response in case of war declaration. In this regard, they are the same.
Also, how about this scenario for instance: if vassal asked for a promise not to attack them, but suzerain feels that he is going to lose control over vassal very very soon, he also can (and should be able to) ask for a promise not to attack them.
And, as far as i know, AI perceives differently when it comes to who asked for "not attacking". If its vassal - it might not affect much, but if it suzerain - AI might react differently.
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