Frankish vowel shifts
I can't actually find any sources about that. I've found vlieke in a Middle Dutch dictionary, but that says it was borrowed from French rather than the other way around. The combination of -iu- and -kk- also bothers me. Normally -kk- is formed from -k- followed by -j- through gemination, but that only applies to light syllables and the -iu- makes it heavy. So it seems that the Germanic form must have been *fliukkijōn. I have no idea how that could have become the Middle Dutch word, though; the -kk- should have been preserved and -iu- normally becomes -u- in Middle Dutch, only -ie- in a few dialects. So maybe it was really *fliukijōn or even *fleukōn.
In any case, I don't think the evidence for this reconstruction is strong enough to warrant an entry. I'd prefer it be deleted.
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