Where does “въ-” come from?

въ- (Old Church Slavonic) comes from Proto-Slavic vъ(n)-, from Proto-Indo-European h₁én — in.

Ancestry of “въ-”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	vъ(n)-	—
2	Proto-Indo-European	h₁én	in

въ- — Etymology, Origin & Word History | Etymology Explorer