The NDAA, unfortunately, funds all the non-combat stuff of the Military on a 1 year basis. Members of Congress who do not vote to pass the NDAA are seen as hating on the military.

Consequently, at the end of every year both sides jockey to jam all their pet projects that didn't get passed into the NDAA. Since it is a Bill, it becomes law.

If the vaccine requirement being lifted is indeed added to this bill then there was likely a compromise somewhere else. Border stuff, DACA stuff, gun control are the top canidates.

Edit: esentially the NDAA passes every year , regardless what is added to it.