you gotta make it all the way the the last area (it's got a lot of dark enemies).
there, you can grind for drops and trade them for Vine. but last i heard, you can't max break her cause you'll be short a copy due to nutaku making an error on that
although quite frankly, you can do quite well without going out of your way for her if you get 40% def down (if you have Amon Unleashed, then 45% is possible)... which is possible if you have the right souls (gawain and d'art). if you have something like 30% or less in def down debuffs, then you might want her and will want to toss her in as a sub eidolon
also, yamata-no-orochi carries a def debuff as well. if you happen to pull her (she's SR as well), you might want to leave some copies of her.