Munada is the noun being called or addressed. When that noun is mudaf, it is mansub in the examples here.
يَا عَبْدَ اللَّهِ
عَبْدَ is the mansub munada and mudaf. اللَّهِ is the majrur mudaf ilayh.
يَا أَرْحَمَ الرَّاحِمِينَ
أَرْحَمَ is the mansub munada and mudaf. الرَّاحِمِينَ is the majrur mudaf ilayh.
A neutral practice model is يَا طَالِبَ الْعِلْمِ: طَالِبَ is the mansub target.