This is the complicated part of the patron/vassal system. Viscount Mongérard is the patron of the Brandt household and at the same time he is the vassal of Marquis Rückner.
Thus it might be easier to understand if one were to call the Brandt household the vassal’s vassal(T/N: yori mago, see explanation at beginning of page to understand) of Marquis Rückner. As one would expect though, there is no such designation such as “yori mago.”