Your question, as it is currently phrased, does not have an answer. You want Frankish halakhic sources from the 8th century?? They don't exist. In fact, good luck finding a Western European halakhic source from before the 10th. While it is true that Jews were purveyors of wine during the reigns of the Carolingian emperors, by the time we get to the turn of the millennium we find it being killed off by a variety of factors - one of which is the ban on gentile wine.
Were Jews unaware of this ban previously? Were those engaged in the trade simply disinterested? Those are good questions, but I don't know that it's possible to find good answers to them.
If you would like to read more about this issue, see Prof. Haym Soloveitchik, "Halakhah, Taboo, and Moneylending", Collected Essays I, 224-236. He sees the decline of trading and the rise of moneylending as inextricably related, and the role of viticulture as particularly relevant there. I quote one section (pp230-231):
Jewish law forbids drinking wine touched by Gentiles. Consequently,
Jews had to produce their own wine - no small task considering the
enormous quantities that were being consumed during the Middle Ages.
In the Mediterranean lands, this was purely a question of manpower -
as the grape is a sub-tropical fruit and grows there naturally. In
northern Europe, however, viticulture is a constant struggle against
the natural environment... When Jews first crossed the Alps or when
any of them first moved into any new location in the temperate zone,
they had to ensure for themselves a steady supply not only of kosher
meat but also of kosher wine. The first was no problem; the second
could be achieved only by either acquiring the skills of the vintners
or forming a close tie with this relatively elite group of workers
with whose accumulated skills, handed down from father to son over the
centuries, no lord could dispense.
Jews in the Carolingian era... were traders, often of luxury items.
They swiftly realized that their necessary skill in viticulture or
their long-standing contacts with vintners could be put to very
lucrative use and, as one way to a ruler's heart is through his
stomach, purveying quality wine could also give them access to power -
so crucial for their physical safety.