I gave Vantage to Alfonse. That was literally the biggest waste of Vantage ever.
I also gave Renewal to Tharja, but then I gave her Swordbreaker so Renewal will never be used. Ever. Though I don't think anybody can really use Renewal. No unit is immune to being ORHO'd by the right hero or combination of heroes.
I suppose one regret I have at the moment was sacrificing my Corrine (F) to get Draconic Aura and Hone Atk. She was only 4*, but it looks like she's a pretty decent unit when you 5* and max her, surprisingly fast and tanky dragon. Ah well. I don't like the character anyway. Tiki (Y) will do just fine.