Uhh..I'll try to go one by one. Just don't take my points too seriously.
Dimitri has great potential with Sword Avoid +20 because of his Personal Skill, which gives him another +20 Avoid when his HP is full. I'd say this is better for a Avoid build rather than Vantage-Wrath because he's more likely to survive instead of taking damage and getting into the 50% HP range with this. Also, he should be using Lances (or anything that isn't a sword/punch) in Vantage-Wrath because of the higher MT and 1-2 Range options it provides, making the Sword Avoid +20 almost 100% useless in a Vantage-Wrath combo. That's a lot of Vantage-Wrath.
Flayn:..I don't think she needs to be able to deal damage. She's a dancer, she can dance people to kill things for her. And the Sword Avoid +20 doesn't really help all that much when you have horrible Spd/Lck. And I think a dancer is really useful even on Warp/Stride strats cause you can just use her to refresh the warper (wait, warp has 2 uses at this game, right?) or just give the Stride to her and open a new combatent slot or just warp her too.
Lysithea: Why?
So yeah, I'm voting for Flayn if support and Dimitri if you want to stack avoid.