Corinthia
In The Iliad, the Catalogue of Ships states that during the Trojan War, Argos, Tiryns, and the other Argive strongholds were led by Diomedes, not Agamemnon, as one might expect. Agamemnon commanded the Corinthian contingent, consisting of Kleonai, Corinth, Sikyon, and (presumably, though not mentioned) Nemea. These states must have been Mycenaean vassals.
So what happened to Mycenae's vassals after High King Agamemnon's untimely death? In other parts of the Bronze Age world, vassals often attempted to and/or succeeded in breaking away when their overlord died; the Hittite emperors and 18th-19th Dynasty Egyptian Pharaohs frequently began their reigns having to subdue wayward vassals. I imagine it must have been the same for Mycenaean kings. Agamemnon's vassals must have broken away after his death, but I can't quite picture Aegisthus as being the warrior type to go out and reconquer them; his hold on power must have been very tenuous. Therefore, it must have been left to Orestes to reclaim his father's territories.
This struggle forms part of the plot for The High King. Expect a mix of raiding, carnage, heavy politicking, and subterfuge fit for a Late Bronze Age king.