Eh, I suppose I'm bored enough to respond to this with a mini essay.
I keep telling you Vaan is not the progagonist.Ashe is. In Matsuno's original concept Basch was supposed to be the main character (old concept art supports this) and Vaan and Penelo were not going to be in the game AFAIK, or at least not as central characters. So feel free to ignore them. I suppose they added them because they were stupid like that - lololol we need more pretty boys because that's what appeals to fans, etc.
There's this conflict about Ashe and her reliance on the Nethicite, and whether or not she will go down the path of a tyrant drunk on power, and basically become the new Dynast King. There's a talk between her and Balthier about this on Phon Coast if I recall. For a while she's somewhat ambiguous in that regard. I would've liked to see her join the dark side, but in the end the spirit of her late husband/boyfriend kept her back. I thought she was a good central character as far as the plot is concerned.
You found Basch's loyalty to Ashe boring, but it is key when considering his relationship with Vossler and more importantly Gabranth. His character is about the conflict between brotherhood and his sense of duty. There's even a fantastic cutscene with him and Gabranth near the end of the game. I dunno, maybe it's just me, but I found it really quite good.
Fran had more development than just bunny-ears and fanservice, but I agree that it wasn't quite enough. However, she did have her moments when you visit her sisters in her woods. You can consider it her own little character arc if you wish. It's actually a rather sad scene if you think about it (she basically abandons her family/tribe for the greater good), but it would've been nice if it was elaborated on a bit more. Still, I felt it was ncie enough for a non-central playable character.
As for why the party is together. I'm pretty sure I remember Vaan having a cutscene (it might've been in the Garif camp) sometime in the story that he wants to find his own way in life. Following the party happens to help him achieve that goal. He is somewhat forced, though, so I'd rather not dwell on it for reasons mentioned above (most notably is that he's actually just a side-character) I don't think it needs to be explained why Ashe and Basch are together. Balthier might've been the guy who "stole" Ashe as she instructed, but he is also drawn to Archades because of Cid, who happens to be his father, if you remember. The talk on Phon Coast I mentioned earlier is exactly about this - he doesn't want Ashe to make the same mistakes his obsessed father did, so he's keeping an eye on her. Fuck Penelo, though, no one cares about her. She probably wouldn't have been in the game in the first place, so whatever.
You fail to mention just how fascinating Vayne is as a main villain/anti-hero. His goal is a noble one, even if he doesn't care about crushing all sorts of opposition to achieve it. I particularly liked his final few moments with Larsa in which it is really quite subtly hinted that he cares deeply for his younger brother and wants him to become a just ruler should he himself fail (pretty sure he left him in Gabranth's care for this reason). It is also notable that he didn't have Larsa assassinated, even though he had the chance to do so. The scene that stands out most to me is when Larsa accuses him of being "wrong", and Vayne simply replies with "Stop me, then.". This is basically telling him that he should stand on his own two feet, and if he wishes to become a strong ruler, he should follow what he believes in, even if it means turning against his own brother (which he finally does before the last bossfight).
That's just my take on it, though. Basically what I'm saying is that it simply caters to a different sort of audience (which you're not part of), but that doesn't mean it was bad IMO. It's rather a simple matter of "to each his own", like with most video games.
/Ray-style wall of text
Edit: Oh, and I don't think it needs to be said but I'm obviously writing this just for the sake of having a good discussion, nothing more.