I understand that vinegar which is deliberately made from alcohol is haraam for one to consume. But is it also najis?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad, sallallaahu 'alayhi wa sallam, is His slave and Messenger.

The scholars differed in opinion about the ruling on making vinegar out of Khamr (any alcoholic intoxicating drink). The view of the majority of the scholars is that it is not permissible to deliberately make vinegar out of Khamr, because the Prophet forbade making vinegar from Khamr. [Muslim]

They also hold the view that Khamr does not become pure by deliberately making vinegar out of it; rather, this vinegar is considered (Islamically) impure.

Shaykh Ibn Taymiyyah said: “If Khamr turns into vinegar naturally without intentionally transforming it into vinegar, then it is a pure vinegar that is made from Khamr and it is permissible to use, according to the consensus of the scholars. The scholars only differed in opinion regarding it, if one deliberately intends to make vinegar out of Khamr.” [End of quote]

For more benefit, please refer to Fataawa 81657.

Allah knows best.