honestly, vivi and cloud are very similar characters in a way. both find out they're not who they think they are and if one considers the compilation part of cloud's continuity, then they're also confronted with an inevitable death.
the thing is entirely how they handle it. people point out that cloud was experimented on to become a pliable puppet, but that's essentially what vivi was made for. the singular reason any black mage is created is so that they could be short-lived, mindless soldiers. vivi is only ~1 year old and he's confronted with the fact that he could have been a puppet forced to obey the commands of his master, and then is later confronted with the fact that he has very little time left on the earth due to how black mages are made. but despite the fact he knows he has very little time to live, he continues to fight the main bad guy with the rest of the party. he gains not only confidence and bravery, but he finds a way to prove his existence (through his 'children' and his fight against kuja). even though it's strongly implied that he died by the end of the game, it's a bittersweet feeling because the player knows that he found happiness in the end. he didn't simply give up when he found out he was going to die perhaps in a few months or possibly even in a year. he kept going. he didn't let the fate of becoming an army automaton befall him. he had the strength and the character to preservere what would be impossible for almost anyone his age (physical or actual).
which is why i can say that without a doubt, vivi is my favorite character in the final fantasy series.