Taking a Hebraistic Vow in the New Covenant

Why did Paul have his head shaved in Cenchreae? What kind of vow was he under, and what does that mean concerning the continuity between the Old Covenant and the New Covenant? There is a correlation between this vow and the end of his second missionary journey, as he decides to go back to his home church in Antioch, sailing from the port of Cenchreae on his way to Syria. But there is another lesson to learn: we must not cut off Christianity from its Hebrew roots.