The Vlie or Vliestroom is the seaway between the Dutch islands of Vlieland, to its southwest, and Terschelling, to its northeast. The Vlie was the estuary of the river IJssel in medieval times. In 1666 the English Admiral Robert Holmes burnt a Dutch merchant fleet of 130 ships (Holmes's Bonfire), that had taken refuge in the Vlie, mistakenly supposing the English could never find their way through the treacherous shoals, so typical for the coastal waters there. Today it's still possible to reach the port of Harlingen by way of the Vlie.
It is often supposed that the old Roman name for the lake that later would become the Zuiderzee: Lacus Flevo, is etymologically related to the name "Vlie" and that perhaps Vlie was once the name of the entire lake and the big river that flowed out of it. In the 13th century large floods widened the estuary and destroyed much of the peat land behind, creating a continuous area of sand and mudflats connecting the sea to the enlarged inland lake and obscuring the flow of the river. When the Afsluitdijk was created, the old streambed from the river to the sea was obstructed.
A river once flowed out to Vlieland (Flyland).
Vlieland is indeed a Wadden Island, now. But once it was connected to the province of Friesland through a large stretch of land.
I think the OLB West Flyland was west of the Vlie/Fly river, and East Flyland was east of that river. But how far to the east reached East Flyland?
Just reread these two lines from the OLB:
Wodin, the eldest, lived at Lumkamakia, near the Eemude, in East-flyland, with his parents.
The navy then sailed to Denmark, where they took on board Wodin with his brave home guard.
Does this suggest Lumkamakia was located in Denmark?