Frankish vowel shifts
Fragment of a discussion from User talk:Rua
You're right, looks like Du. vliek is actually a reconstruction from OFr., but it does have obvious Frankish origins, with many cognates, so it shouldn't be outlandish to reconstruct. http://gtb.inl.nl/iWDB/search?actie=article&wdb=ONW&id=ID3683&lemmodern=vliek