Pearl island was known as O'ahu Island before the war, but quickly got its name changed to the more easy sounding name of Pearl Island, mainly because of the Aloha Island's Capital.
'Pearl City' is located on the remains of an old Navy harbor there. It was the only island that got hit by an atomic bomb, namely the city of Honolulu got hit, a crater still remains in the center of the city. Now the city bears a close resemblance with the usual cities on the mainland of the US. North of the city, on the main road connecting the Northern coast with the southern coast, lies the entrance to the only Vault on the archipelago. Vault 4. A large majority of citizens in Pearl City are descendants of Vault 4. A bunch of other settlements are on the island, whom have barely any connection with the city.
This Vault is the only official Vault-Tec Nuclear Fallout Shelter present on the islands of Hawaii. Located on Pearl Island, formerly known as O'ahu it was built near the main road that connected the North and South coast of the island, incidentally the most traveled road on the island. Far enough from Honolulu to not get completely obliterated by crippling heat and yet not too far away from the city so that it was possible to get there in time, a mere five minute drive.
Vault 4 was not planned to have a social experiment, its great distance from Mainland US soil would make data collection almost impossible in case of an all-out nuclear war. Instead is would do exactly what the brochure said it would, having the normal capacity and standard accommodations. Not so much for the benefit of the potential Vault-dwellers, but as a way to keep show the United States that Vault-Tec would build enough vaults for everyone and all across the nation. Essentially Vault 4 was build to reassure the citizens and holiday-goers of Hawaii that Vault-Tec was there in case of an emergency.
Interestingly enough, Vault 4 was one of the few Vaults that did not require an entrance fee or prerequisite psychological tests. It would be free for everyone until the limit of a five-hundred people was achieved. Then the doors would close automatically, regardless of family or rank.
Dudeville on the Northern shoreline is a settlement of surfers who dig the big waves caused by the anomalies in the weather. They are mostly harmless Anarchists whom bother no one. In fact they are mostly avoided by others because they are slackers of no value what so ever.
Laie is a small settlement based in the ruins of the town of the same name. They are of native Polynesian descent who've survived somehow and follow old world values. They don't allow outsiders in for anything else but business. And mostly don't trust people from "the city"
Trigger Happy Son'sabitches!
Trigger Happy Son'sabitches! is a raider gang or something that comes the closest to it, a blight on Pearl Island they steal and enslave whomever they please. They sit on a large ammo stock left behind on the deserted Marine base of MCBH, they wish to improve their stock by claiming Pearl City, who's Pearl Harbor storage serves as a deterrent against these marauders.