There is a very interesting Flavian genealogy found in the following article:
It is also reprinted in the following book:
"A History of Rome" (3rd Edition, p 260) by Le Glay, Voisin, and Bohec.
The genealogy names the parents of Vespatian (and his brother T. Flavius Sabinus, prefect of Rome) as T. Flavius Sabinus and Vipsania Polla. The elder T. Flavius Sabinus is further named as the son of T. Flavius Petro. Vipsania Polla is named as the daughter of Vipsanius Pollio.
In previous posts, I proposed that Vespatian and his brother were in fact the two older brothers of Caligula who had run afoul with Tiberius Caesar, were disgraced, and presumed killed. Caligula and his older brothers were the sons of Vipsania Agrippina and Germanicus.
It is not such as stretch then to equate Vipsania Polla with Vipsania Agrippina, or make Vipsania Polla an adoptive mother. Vipsanius Pollio would also equate to Vipsanius Agrippa (dynastic fellow of Caesar Augustus).
Likewise, T. Flavius Sabinus (father of Vespatian) would equate to Germanicus and his father Titus Flavius Petro to either Tiberius Caesar (adoptive father of Germanicus) or Drusus (actual father of Germanicus). Tiberius and Drusus were the two sons of Drusilla by Tiberius Claudius Nero. (Note also the apparent paternal link between the name Petro/Peter and Gemellus the grandson of Tiberius Caesar.)
The name Flavius is a variant of Fulvia (both are defined as "blond"), who was first the wife of Publius Clodius and then of Mark Antony. By Mark Antony, Fulvia was considered the mother of Iullus Antonius. However, as the (legal or actual) son of Clodius, this same son appears to have instead been called Tiberius Claudius Nero. The lineage of Titus Flavius Vespatian thus derived from Vipsanius Agrippa on the maternal side and from Fulvia on the paternal. This conclusion seems far more reasonable than the present academic position that Vespatian and his brother were plebs that somehow rose up to become Caesar and Mayor/Prefect of Rome, respectively.
© Charles N. Pope, US Library of Congress. All rights reserved.