The biblical source for the variety of statements that count as full or regular vows, developing the "hekesh," a linkage, between nedarim and nezirut. But why require the hekesh at all? This particular connection seems evident in the biblical text regarding nedarim anyway. This hekesh relies on the phrasing of the Torah being presumed to align with the way people speak. Also, the two Torah prohibitions that one violates when one breaks a vow. And again, how nedarim align with nezirut with regard to these violations (where nezirut seems to only have one of the prohibitions, not both).