To take the final limit of every player you have to complete all the previous levels of limit,that means you have to have take level 1,2 and 3 of tifa for example to accept "final heaven"-which is pretty good believe me-,that's how a player can learn his final limit.
About the vincent's weapon,what do you mean that the submarine don't let you?Cause i think only with submarine and gold chocobo you can take it.