A more in-depth review of the Vita Germani can be viewed in this paper entitled, A Contemporary View of Vita Germani.
Here is a basic review of the subject.
In his work entitled “The End of Roman Britain: Continental Evidence and Parallels”, Professor Ian Wood provides a strong argument against the traditional years of 418 to 448 for the episcopate of Bishop Germanus. He suggests that Germanus became the Bishop of Auxerre in either 407 or 412. Comparing the Life of St. Germanus, Bishop of Auxerre, by Constantius of Lyons (trans. F. R. Hoare, The Western Fathers) to other contemporary sources, the latter of the two years has been chosen for Ambrosius Aureliani. It should be noted that F. R. Hoare accepted the traditional years for the episcopate of Bishop Germanus.
In section II, Constantius tells that the populace – clergy, nobility, townspeople and country folk – demanded that Germanus was their bishop. The sentence that follows it states that a war was declared by the people against their magistrate and they overthrew the official. The line seems odd at first glance but it echoes the words of Zosimus quoted in An Age of Tyrants. The cited passage told of Roman officials being expelled from Britain, Armorica, and other Gallic provinces around 409. Four men – Constantine, Attalus, Maximus, Jovinus – tried to usurp the Western Empire during the years 407 to 411. Each had magistrates and military personnel, giving rise to several sets of traitors during this time period.
Four interpretations can be made about the ascendance of Bishop Germanus. The first is that the magistrate stripped from his office was not Germanus and the election of Germanus was at the time that this civil war raged. Secondly, Constantius artistically expressed Germanus’ ascendance and it had no historical connotation. Or alternatively, Germanus was a magistrate in one of the usurpers’ governments. Removed from office but spared by popular-consent, the divine [imperial] authority conscripted Germanus to an ecclesiastical office. This view would accounts for his compulsion to receive the religious position. This would, also, explain why Constantius fails to note any of Bishop Germanus’ deeds as a duke. It would be difficult even for a talented orator to honorably mention exploits against the Empire. With the fourth possibility, Germanus was an Honorius-elected magistrate and removed by rogue members of society.
Sometime after becoming the Bishop of Auxerre, a plague hit the region. With a reevaluation of Independent Britain by Gildas, there is a possibility that the plague that hit Britain happen around the same time.
Though lacking the year for the plague that Constantius mentioned, it had to occur in between 412 and 429. The end date is based on the bishop’s first documented involvement in British affairs, which began in section XII ran through section XVIII. Much, if not all, these sections occurred in 429 when Prosper of Aquitaine notes Germanus’ trip to Britain. (trans. Alexander Callander Murray, From Roman to Merovingian Gaul).
The three references to miraculous powers in sections XX, XXI and XXII are dated to 433 by the entry in the Gallic Chronicle of 452 regarding Germanus (trans. Alexander Callander Murray, From Roman to Merovingian Gaul).
In section XXIV, Auxiliaris governed as the Praetorian Prefect of Gaul and warmly received Germanus when he arrived in Arles. Hoare places Germanus’ visit between the year 435 and 439 based on his note for section XIX. There doesn’t seem to be a reason not to accept this time period.
In section XXV, news of the Pelagian heresy troubling the British reached Germanus at home. This time, he traveled to the island with Severus, the Bishop of Trier. By prior points and with future considerations, the second trip to Britain happen between 435 and 441.
Returning from Britain in section XXVIII, Bishop Germanus confronted King Goar as his Alan tribes and cavalry filled the roads, ready to subdue Armorica. The Gallic Chronicle of 452 places this event around 441/442. The bishop went to Italy seeking a pardon for Armorica. Constantius doesn’t give the impression that six years had elapsed between Germanus’ trip to Britain and the one to Ravenna.
In section XL, Tibatto incited the people of Armorica to rebel, again. This event does not contradict the events of the Bacaudic revolt described in the Gallic Chronicle of 452. Though said to be captured in 437, the Gallic entry does not specifically state that Tibatto was killed like other rebel leaders were. In fact, trouble continued for another eleven years. After being implicated in the Bacaudic revolt, Eudoxius fled to the Huns in 448.
In section XLII, Bishop Germanus died while in Ravenna.
So, Bishop Germanus’ expulsion from office and his conscription to the see of Auxerre fits neatly within the chaotic times of 412 while his involvement with the Alan king, Goar, establishes 442 as the end of his life.