Is there anything in canon of the history of the Order, of Magi and/or Apprentices being kidnapped by Viking raiders?
Maybe they were mistaken for priests, or someone else intellectually skilled but harmless.
Could the descendants of these captured hermetic wizards have formed a magical co-operation pact, which gives rise to the Order of Odin?
F&F p.67 tells the story, how by 850 "the Flambeau magi of the region (scilicet: Gascony) incorrectly believed that the so-called Order of Odin had reignited a war through the sudden series of raids on the western coast."
About 840 an isolated magus or two from that area, unrelated to these Flambeaux, might have also been kidnapped, if that helps your saga.
That is an interesting question.
When did the Order of Hermes begin to think there was an Order of Odin ?
By F&F p.67 referenced above, that would be Kaeso and the Flambeaux of Gascony around 850. "They mistook the individual vitkir supporting their clans and relatives for members of a larger organization, all sharing an aggressive and unified vision, finding an enemy where there were merely raiders. Wrongly convinced of the scope of this danger, Kaeso returned to Bordeaux to fortify his temple with the full support of House Flambeau and his Hoplites."
I really should get a copy of F&F someday.
Still, you said 850 was considered re-igniting the conflict with the Order of Odin, so I thought that meant there was an acknowledgment well before 850.
There was a bit of a misunderstanding.
Well, considering that the Lindisfarne raid was in 793 the OoH would not really have been aware of the Order of Odin before that. The peak Viking age was the 9th Century, especially after the end of the reign of Charlemagne. I would not see 850 as re-igniting the conflict, but rather the peak of the conflict, since 845 consisted of one of the sieges of Paris by the Vikings.
Not much earlier. The earliest tmk is at the Grand Tribunal in 832.
You can look up GotF p.113 box The Order of Odin, HMRE p.121 box The Order of Odin and in AM p.133ff The Order of Odin, there especially p.135 The Viking Invasions:
Starting in about 830, the Vikings led a major assault against Ireland, <...>
It was there that the Scandinavian rune wizards first encountered representatives of the Order of Hermes, in the form of a group of Diedne magi from a covenant several miles northwest of their camp. <...>
the magi realized that they were unusually vulnerable to these attackers, and
they were troubled. They brought word of these strange northern wizards to the Grand Tribunal of 832, where those who had heard of their pagan faith and their Anglo-Saxon kin dubbed them members of “The Order of Odin.”
In 837 a delegation of primarily Irish magi from many different Houses, led by Máel-tuili of Merinita, sought out the rune wizards and attempted to parley with them. At first, the Viking leaders were abusive and demanded to know their secrets, but after hours of negotiation the rune wizards eventually agreed that
they would leave in peace if the magi paid to them a tribute of 3,000 pounds of silver and forty pawns of vis. This price was arranged, but soon after they had it in hand the Vikings reneged on the deal and attacked. The leaders of both parties fell in the subsequent battle, and many other magi and wizards were killed. The news of this betrayal stirred equal parts of outrage and horror in the local magi and throughout the Order, but no immediate action was taken.
But you should read F&F p.67f Castra Solis completely, indeed. Here's what I consider the central part about when the Hermetic legend of the Order of Odin really took hold:
Between 840 and 850, attacks on local villages increased, and certain isolated
magi living in the interior of Gascony complained at Tribunal. By 850, the Flambeau magi of the region incorrectly believed that the so-called Order of Odin had reignited a war through the sudden series of raids on the western coast. They mistook the individual vitkir supporting their clans and relatives for members of a larger organization, all sharing an aggressive and unified vision, finding an enemy where there were merely raiders. Wrongly convinced of the scope of this danger, Kaeso returned to Bordeaux to fortify his temple with the full support of House Flambeau and his Hoplites. <...>