A.T. Robertson Commentary Ephesians 4:17

"This I say therefore, and testify in the Lord, that ye no longer walk as the Gentiles also walk, in the vanity of their mind," — Ephesians 4:17 (ASV)

That ye no longer walk (μηκετ υμας περιπατειν). Infinitive (present active) in indirect command (not indirect assertion) with accusative υμας of general reference.

In vanity of their mind (εν ματαιοτητ του νοος αυτων). "In emptiness (from ματαιος, late and rare word. See Ro 8:20) of their intellect (νοος, late form for earlier genitive νου, from νους).