This is a really interesting account, and one that's not well known. St Germain of Paris died in 576 and was commemorated shortly thereafter by his friend and contemporary, the famous poet Venantius Fortunatus. Historians interested in St Germain look there, rightfully concluding that nothing further about his life is included in the Miraculi Sancti Germani, which was written by Aimon, a monk living in the monastery of Saint-Germain-des-Prés, which housed the relics of St Germain. Aimon wrote these Miraculi after 885 but likely before 896, the date of his last datable work that survives and a reasonable guess for the year of his decease.

Aimon's account is a fantastic story, most recently edited in 1852 and never translated into a modern language (at least insofar as I could find). It consists of nineteen chapters, the first thirteen of which concern Ragnar's raid on France in 845; the remainder seem to consist of healing miracles and the like, although I've only given it a quick skim. Aimon's account has the vikings assault Paris, disturb Germain's bones, flee from a miraculous fog, die of dysentery upon their return, and the Danish King Horik then free the surviving Christian captives.

This account, written a full generation after the events they describe, varies significantly from the Annals of Xanten and the Annals of St-Bertin, both of which were written probably right around 845. The chronicler of the Annals of St-Bertin recorded that Charles the Bald had paid the viking raiders off. The chronicler of the Annals of Xanten gave a similar account.

I'm not entirely sure what to do with Aimon's account, although it certainly is something I'll need to deal with in my own scholarship—so thanks for bringing it up! My first reactions are two-fold: (1) Aimon was likely responding to a particular need within the community of St-Germain; perhaps the dilapidated state of St Germain's relics and tomb needed to be explained, or perhaps his community just needed a boost of faith, and these events were just distant enough that they could be reshaped into the kind of stimulant that Germain's cult needed. And: (2) this was a time when stories were circulating about missionaries to Scandinavia like Ansgar and Rimbert; these missionary-bishops were famous for having traveled north and ransomed slaves in the markets of Ribe, Haithabu, and Birka; perhaps this was a similar response by Aimon and the community of St-Germain to concerns about the booming slave trade of Christians getting plundered by vikings and sold south through Scandinavian markets.

So in sum, I'm not really sure. Historians interested in St Germain haven't dealt with it much, and those interested in the Carolingian period have a ton of other sources that seem more important than this one, so it's gone under-studied. I've given a few thoughts on where I might take this in my own work, but I'd be happy for any other leads or questions that might help strengthen the depth of my own scholarship on the matter. Welcome to the historian's workshop!