Unlike most characters, Vincent only has one limit per level. To unlock the next level, get killing blows on enemies. Killing blows are cumulative over a save file and DO count before you reach the level.
Using the in-game cheats is allowed, so you can turn on Battle Assist to speed up using the limits. A good place to farm kills early is the forest east of Junon, as you can reliably get 3-5 enemies that can be taken out with Matra Magic or a basic magic with All materia. Late in the game, the forest at Mideel has 3-4 that can be taken out with heavy-hitting Enemy Skills and also doubles as a decent AP farm. The exact numbers needed are below.
Death Gigas is unlocked at 40 cumulative kills, and Hellmasker is unlocked at 96 kills.
Chaos is obtained at a hidden cave on the Western Continent. You can reach it in one of three ways:
- Green Chocobo: Run over the mountains west of Gold Saucer.
- Blue Chocobo: Go to the river south of Corel Prison, then run up the river and up the waterfall.
- Submarine: Head north of Costa del Sol and look for a break in the sea wall. Once on this path, take the first left and look for a tunnel; follow it to the end and surface.
Put Vincent in the party and go into the cave for some scenes. Leave the cave, and now you'll have to find ten or more battles. There are no random encounters here, so you'll have to leave and come back once you're done. Examining the new object in the back where Lucrecia was gives you Death Penalty and Chaos.