Complex sentence: Louis' son Pepin, now King of Aquitaine, stamped out the Vasconic revolt in Gascony and then pursued the chieftains who had taken refuge in southern Vasconia, i.e. Pamplona and Navarre, no longer controlled by the Franks.
Split into simpler sentences: Louis' son Pepin, then King of Aquitaine, stamped out the Vasconic revolt in Gascony. He next hunted down the chieftains who had taken refuge in southern Vasconia, i.e. Pamplona and Navarre, no longer controlled by the Franks.