Cousin of Domitian
Vespasian Junior was the son of Flavius Clemens and Domitian Junior was the son of Flavia Domitilla and therefore where his cousins. Domitian had no heir and as such toward the end of his reign he adopted these two boys. Domitian feared fearing the legitimacy of rival heir Titus Flavius Clemens, who was the nephew of Vespasian and husband of Domitilla. In 95AD, Domitian executed Flavius and exiled his wife to Pandateria on charges of atheism. It is possible that they had becomes Christians. As nothing further is recorded with respect to the two boys. Coinage struck in Smyrna (Ionia) (modern day Turkey) exists but only with Vespasian Junior, not his brother.
Æ17 Bust of Vespasian, Jr., facing right
The Monetary History of the World