I think 128 kills have the same effect as 1 attack point. Also, I don't think you need to do it while having the Death Penalty equipped, any kill Vincent ever does should do.

However, I think overflow is way way way easier to achieve with Barret. Just put materia with really high AP (10000 AP for barret have same effect as 128 kills for Vincent) in his weapon (KotR, Hades, the W materias, .....) and you should easily be able to achieve an overflow. Check out this video:

YouTube - Final Fantasy 7: Vincent AND Barret Overflow Glitch!!