Tidus fit very well into the story of FFX though. He wasn't just "along for the right", though I guess you could argue that.

FFX was Tidus' story.

Vaan really has no place in the story, he would be much better as a character you just aquire along the way, then again I haven't played through much farther than yourself, for all we know he might just not be fully developed.

Oh and I must add, so far FFX is superior to FFXII when it comes to story, but FFXII is superior when it comes to gameplay. I absolutely hated how battles in FFX were pretty much like Rock->Paper->Scissors. It gave a use for every character, such as Wakka being the master of killing flying monsters, Lulu being the master of killing elemental monsters, but it was absolutely horrible in my opinion. But FFX's story is absolutely amazing.

I must also add, I've taken a liking to the word "absolutely" it seems. I said it 3 times in the past paragraph.