Function secondary_key 

pub fn secondary_key(
    key: &Key,
    secondary: &str,
    sixth_minor: Minor67Default,
    seventh_minor: Minor67Default,
    case_matters: bool,
) -> Result<Key>

The key a secondary numeral establishes.

music21 reads the numeral after the slash as a chord of its own and takes the key from that chord’s root and quality, so the vi of V/vi in C minor is the raised sixth degree the same way a plain vi would be.