Vaan is not nearly as bad/annoying as he's made out to be. Part of the problem is he's very clearly a shoe-horned in protagonist. Which isn't all that bad. But part of the problem is also his ENG voice acting just doesn't... super work right? There's something about English voice actors trying to play the role of the Japanese plucky teen that never really fully works, especially from 15+ years ago. They often aim to copy the original Japanese voice acting's intonations and it just ends up sounding kinda whiney. (Definitely some Tidus-syndrome going on with Vaan.) Played 12 again for the first time in ages not that long ago, but enabled the JP-audio and it made Vaan a lot more bearable protagonist to listen to. Which is a pretty stark difference that sticks out when the rest of the ENG voice acting is generally pretty decent. It's also from the era of dubs before Square-Enix started matching lip flaps separately for the ENG voices, so it's often awkward to see/hear the English dialog paced to fit Japanese mouth-flaps.
I think I prefer some of the ENG voices to the Japanese ones, but I think on the whole the JP voice acting is just better and better reflects the characters and writing:
Ashe's voice acting is not bad, but rather dull in English, when her JP counterpart tends to imbue a lot more emotion into her line reading which fits her personality as construed in the dialog and plot as a passionate, but desperate and lost leader a lot better.
Penelo's voice actor in JP gives her character a very stereotypical "older sister" personality in the way she intonates that matches her personality and relationship with Vaan a lot better, in a way that kinda doesn't really exist in English.
I actually prefer Fran's ENG voice acting because the accent they give her helps immediately situate her as an outsider, and her actress does an awesome job/gives Fran a lot of personality. But that gives her character a completely different flavor/impression from the JP actress who doesn't really put on an accent, and just has a very sultry, mature, tone to her voice. (If you haven't played FF12 in Japanese, but understand the point of reference, she's acted by the VA that did Myung in Macross Plus.)
Balthier and Basch's actors in ENG both knock it out of the park, but the JP actors for both are *huge*, incredibly famous talents that lend the roles their gravitas in the way that the English actors really can't through no fault of their own.
Vaan's role compared to other FFs is closest to Tidus, who also tagged along on someone else's epic quest, except Tidus does turn out to be central to the story because of who Sin is and where he came from.
Tidus has personal connections to what is going on, and early on he seems like he's just tagging along/an observer. And both act as surrogates for the audience so that things neither understands can be explained to the audience through dialog more naturally, and as outsiders to give the others perspective. But he actively involves himself in the story in ways Vaan never does. Very quickly, Tidus becomes the emotional core of the group - keeping their spirits up, pushing people forward, and stepping up to be a real leader of the party when the others are in emotional turmoil or out of commission. And as he proclaims at the very beginning of the game, this story is his story and about him. 12 stops being about Vaan almost immediately. I'd say the best analog for Vaan in how he's used in the story isn't Tidus but maybe more like any of the more primary characters from FF6 like Locke or Terra. Both are important but neither is really the focal point of the story or actively drives the plot forward in the way that say Celes does.