I like Vaan, i saw him as a blank canvas. I made him extremly diverse, making him my trump card that could fight in all areas. Which is how he is shown on dissidia. While the other stuck with their original weapon class.
At the very least, you can see why he wanted to continue to fight with the others. Penelo on the other hand, was thrown into this massive event and only stuck around because of Vaan. She is way more pointless.
I dont understand how Vaan could see the prince of dalmasca's spirit though. I kinda just thought that the writers were trying to intergrate into the story more.