The fact that the Phoenician Vau was retained in the Greek alphabets, and the vowel v added, shows that when the alphabet was introduced the sound denoted by was still in full vigour.
With Vau it was different; the u-sound existed in some form in all dialects, the w-sound survived in many far into historical times.
Most authorities have held that the new form was derived from E by dropping the lowermost crossbar; some have held that it developed out of the old Vau, a view which is not impossible in itself and has the similar development in Aramaic (Tema) in its favour.