Vathlo Island is populated by a "highly developed black race" of Kryptonians, and "retained its independence throughout history and did not join the planetary federation, though good relations Home of were maintained".
Black Kryptonians were first shown in Superman #234 with the island itself shown on a map in Superman #239.
Vathlo was rarely if ever referenced beyond these few issues, although a black Kryptonian named "Iph-Ro of Vathlo" appeared in Superman: the Man of Steel #111.
An offhand reference to the island was made in Alan Moore's story "For the Man who has Everything", where "racial trouble with the Vathlo Island immigrants," are mentioned in a dream world Krypton that had avoided destruction. It is believed, based on the appearances of black Kryptonians in recent Superman issues, that the Vathlonians eventually were integrated into Krypton proper.
Most recently Vathlo has gained new notoriety by being the homeland of Superman in earth-23.