Often assumed to come from the zero-grade. However there are no secure examples of *n̥ > Balto-Slavic *un (other examples like*sъto (often considered to be an Iranian borrowing) are all riddled with difficulties and unreliable), and the o-grade*h₁ón can be posited instead. See also*mъnogъ,*vъnukъ,*vъnъ,*vъtorъ and*kъ(n),*gъnati,*gъrnъ,*gъrdlo,*kъrma,*dǫti/*dъmǫ.
Because of the law of open syllables, the final-n was normally dropped. But when combined with a stem that (originally?) began with a vowel, it was attached to the following word. (Example: *vъn*jejь >Bulgarianвнея "in her").
Anikin, A. E. (2011), “в, во”, inРусский этимологический словарь [Russian Etymological Dictionary] (in Russian), issue 5(буба – вакштаф), Moscow: Znak,→ISBN,page304
Vasmer, Max (1964–1973), “в”, inOleg Trubachyov, transl.,Этимологический словарь русского языка [Etymological Dictionary of the Russian Language] (in Russian), Moscow: Progress