What does the term the vainest knife mean in the song Moody River?

Moody River was written by Gary D Bruce and first recorded by him in 1961 using his performer name Chase Walker. His original Words were "more deadly than the SHARPEST knife" but during the recording session the P in "sharPest" kept popping so rather than find a different mike, Gary (Chase) changed sharpest to "VAINEST" on the spot and probably very little if any thought went in to the meaning of vainest.

When Pat Boone recorded it a couple of months later Dot Records chief, Randy Wood checked to make sure the lyric was correct before releasing the record. Pat Boone says he had no idea what a "vainest knife" was and just sang the lyrics as written. Pat's Moody River topped the charts on June 17, 1961.