How have they vanished? I don't think they've vanished at all.
Can't we simply say the Franks evolved as a culture over the hundreds of years since their emergence as a barbarian tribe, to a ruling warrior aristocracy over a predominantly differently cultured (gallo-romanic) population, to finally a hybrid nation?
I mean look at the evolution of the Normans into Anglo-French then ultimately English. Their situations are actually pretty analogous. Invasion, replacement of the ruling caste. After hundreds of years, cultural integration to form a new hybrid culture with a hybrid language. End culture does not look much like the first culture, but maintains a continuous lineage.
Critical question, how are you defining vanished?
Are you talking about genetic lineage? There is a continuous genetic lineage of French (as well as most of europe)'s descent from charlemagne.
As a unified state? One could argue a non-bureaucratic kingdom could only be held together by the strength and will of the king by rule of conquest or the threat of it. Without which a weak king naturally devolves administration to his vassals with their hereditary claims, which results in feudalism. In which case, a unified state is the anomaly, not the norm to regress to.
As an ideology? Early frankish history and culture is celebrated as an integral part of medieval and modern france.
I disagree with your assertion that the Franks have "vanished." They have merely evolved like all other civilizations evolve. Frankish->French. Just like Norman->AngloFrench->English.
Just like the culture of the Rome of 14 AD was not the same as the Rome of 285, 476, 1204, and 1453.