Ok, I read some formulas and crunched some numbers.

If you work it out 2048 (128*16) kills is equivalent to x1 damage. So if you have less than 2000 kills with Vincent you will do less damage than having a normal weapon equipped. At 65,000 kills Vincet will do ~32x normal damage.
According to my math at lv99,255pwr and 53,000 kills Vincent should be able to Overflow Emerald Weapon.

Under the same circumstances Barret would require 4,120,000 AP. 8xKotR is only 4mill so you would need to take advantage of Hero Drinks or Berserk+Critical Hit to pull it off on Emerald Wep.

Basically with four hero drinks you need Half of the usual AP or Kills. I'm also using 255 Pwr, which requires quite a bit of source farming.