As a side note, funnily enough Terra is just as bad as Vaan in the 'contributes nothing to the main plot' yet no one ever mentions this.
In all honesty, I probably should've picked a better example like FFVI. That game, while ostentatiously granting Terra the role of MC (for the ending credits and everything thereafter), didn't really grant it to her alone but also Celes and Locke, both of whom took control of the plot at certain points. Granted, Terra was more a demonstration on what would happen if the mysterious waif female lead was actually the MC for once instead of just another damsel in distress, but she still got distressed on more than one occasion and had to leave the party for a goodly chunk of the first half and was wholly optional for the second. Vaan at least, never had to leave the party (no matter how much we'd rather have Fran or Ashe as the party leader in cities and villages).
Though on that note, Cloud also left the party for a brief stretch, leaving Cid (and Tifa I guess) to pilot the party for a while, but that's closer to a brief reprieve compared to Terra's absence, especially since getting him back in your party was plot mandatory (compared to Terra's "I must go now for the ending sequence needs me.").
You were saying?
Imma let you finish, but Warrior of Light is the most story irrelevant FF character ever.
Who the hell plays a team with a Warrior in it? Its 4 White Mages all the way (to White Wizards).
Admittedly I'm guilty of not playing the original two. I had the FF Origins disk in High School but really just couldn't get into either. Maybe I should at least give the first one a go someday. I hear 2's just pretty much abusing the stat system for an hour then making cake of the rest of the experience.
Biggest mistake you can make with FF2 is grinding your HP up. Late game and onward, undead enemies start showing up in droves and their attacks always drain health based upon a 1/16th percentage of your total health per Hit (and this is on the old NES style Hits based system where higher speed means more Hits per attack), and between that and the fact that Cure Magic have a hard time healing more than 1500 HP per cast at level 13-ish, which itself is probably the highest level you can expect your Cure Magic to reach by the Final Boss Fight with some grinding, you really can't have your characters keep up with that kind of damage (especially since Level 13 magic costs 13 MP per cast, its not particularly economical to go around burning MP when fighting 6 or 8 undead enemies throwing out 1000s of HP of damage per individual undead enemy's attack).
What you should do is focus on having your designated Tank be the only one in the front row, and have him/her dual wield Shields or a Shield and a Dagger or something that boosts Agility, because Evade is what you want since it cuts down on the number of Hits per attack that connect. Give your other two characters either a Bow or make them your designated Black Magic user, and make sure the Tank gets Blink cast on him/her a lot.