In 78 CE, the Parthian king Vologases I died. He was succeeded by Vologases II, who appears to have claimed that he was Vologases' son, and by his brother Pacorus II.
Vologases II is only known from his coins, which were minted in Seleucia on the Tigris, the capital of the Parthian Empire. The oldest of these can be dated to 77/78, which suggests either a revolt or a co-rule. In the second scenario, Vologases II was recognized by Vologases I and may indeed have been the son of the earlier king.
However this may be, the last coins were minted in 79/80, which suggests that Pacorus captured Seleucia. In any case, it was Pacorus who was to rule the Parthian Empire for another quarter of a century, until 105. Towards the end of Pacorus' reign, a king namend Vologases III would rise in rebellion, claiming to be a son of Vologases II.
The chronology of the Arsacid kings of the Parthian Empire is less well-understood than, for example, the sequence of Seleucid and Ptolemaic kings or the emperors of Rome. This information is based on the researches by G.R.F. Assar, as published in "Iran under the Arsakids, 247 BC – AD 224/227" in: Numismatic Art of Persia (2011).