I tested Vanda extensively in beta, very useful on offense but I think she’s a weak defender. Her damage is very mediocre and her vampirism is easy to play around (either wait it out or cleanse it with an indirect healer like cRigard). I actually lost cups with Vanda over Mitsuko flanking Guinevere.
J.F. burns all enemies and protects your Heimdall from defense drops for much longer turn duration (4 vs 2). Personally, I think J.F. is the better defender.
I’m not sure on Garnet flank though – but with two healers in the center (Heimdall and Garnet) you might be risking a passive center and giving the attacker more room for charging skills.
RE: Garnet vs Vanda. Personally, I think Garnet is the better hero. Her HP boost is more valuable than the small amount of damage to all Vanda deals, and you can hold Garnet for a longer charge if you don’t need her skill to also get more benefit.