I don't think anyone knows the total Crit+ cap. You won't hit it with just Vanya I know that. Maybe with AF2 + Allagan + AF accessories.
However, the equipment itself has a Crit+ cap. I just learned recently to watch for +0 during melding. That can be an expensive lesson. Just below the Materia at the top of the Meld window, you will see a confirmation that you are getting the +4 or whatever from your Materia meld. If that number shows +0, or any number less than the +# on the Materia you are melding, then you have hit the cap for that stat, for that piece of equipment.
OP, I get the feeling you know that already. Just mentioning for others who maybe will read and won't have to learn that the hard way.