This is the answer Imam Rassi society gave (A Zaidi group).
Q] In Zaydiyya is Wilayah of Imam Ali (عليه السلام) considered part of Usul Al Deen? If it is, does this mean someone is not mu'min and does not go to heaven without believing in it?
A] wa alaykum as salaam wa rahmatullahi wa barakatuhu!
Thank you for your question! This one is a bit more difficult to answer. The earliest Zaydi imams held to the belief in the wilaya as a precondition of belief. Imam al-Haadi (عليه السلام) said in his Ahkaam very explicitly that the term 'belief' cannot be applied to one who doesn't accept the wilaya.Imam ar-Rassi (عليه السلام) has a whole treatise on the obligation to believe in the wilaya. We also have statements from Imam Ja'far as-Saadiq (عليه السلام) in which he was asked if rejecting the wilaya makes one a disbeliever and he replied in the affirmative.
However, the difficulty arises in the actual application of this. No imam or scholar explicitly says that such person would go to the Hellfire, nor are any of the Islamic laws waived from such person, such as adaala in testimony or leading the prayer, etc. So it would seem that the precondition of the wilaayais moreso theoretical than practical. However, what that means, I don't know.
And Allah knows best!