Dear Vanitas X Aqua fanfic writer.
I'm going to ignore how short your chapter's are(You could have combined both chapters and made it a one shot.) and just talk about the story itself.
First of all is the characterization. Aqua is serious, kind, strong willed, and responsible, as she is the most reliable of the three. She is always worrying about Terra and Ventus, who she viewed as her brothers. She has been shown to be blushing and embarrassed when Zack asks her to a date, and starts shaking her head and arms, attempting to compose herself. Aqua would probably not admit to Ven that she liked it when Vanitas kissed her. Even if she had feelings for him I doubt she would admit it to anyone including herself.
Also Vanitas is not a fluffy bunny. As an Unversed who feeds off of negative emotions and a manifestation of pure darkness, Vanitas is proud, cunning, cruel, and pure evil. The lives of others hold no value to him, and he would kill without a moment's thought. His overall personality is in an extremely stark contrast to Ventus and, by extension, Sora, Roxas, and Xion who are kind, gentle, valiant, and cherish friends and posses heroic qualities.He is the male version of Larxene.
Seriously look at these videos and see how they act around each other
http://www.youtube.com/watch?v=-fe5rimAzAItranslation
Aqua: “You! You’re the one with the mask Ven was talking about.”
Vanitas: “Oh? Ventus? How is he? Has he gotten stronger?”
Aqua: “What is that supposed to mean?”
Vanitas: “I’m the one who is asking the questions. And even if you knew what I was talking about, it wouldn’t help you. Since you’ll be gone in a second.”
Aqua: “I’ve had enough of this!”
After the fight
Aqua: “Who are you?”
Vanitas: “Ahahahahahaha”
“You’re quite good. Okay then, you’ll be a spare.”
“They must get stronger.”
http://www.youtube.com/watch?v=FUmgHhVXNKE translation
Vanitas: “Was it fun looking for treasure?”
Aqua: “When did you...”
Vanitas: “He no longer needs a toy like this.”
“So... a spare is no longer needed.”
Aqua: “Bastard!”
http://www.youtube.com/watch?v=66HdBvtj3RY translation
Braig: “You keyblade wielders sure are strong. Looks like I wasn’t wrong in my choice. Well, I’ve wasted enough of your time.”
Aqua: “What!?”
“Ven!”
Ven: “Aqua!”
---
Ven (memory): “It’s because you’re my friends I want to ask you. Please... kill me.”
---
Mickey: “Good, you’re awake.”
Aqua: “Ven!”
“Oh good, Ven. You’re alright!”
“Ven?”
Mickey: “He isn’t Ven!”
Vanitas: “That’s right, I’m not Ventus. I took his heart. With this X-Blade I’ll open the door, and connect all the worlds! And the hearts of light will seek the Kingdom Hearts. And keyblade holders from all the worlds will gather here. And the legendary Keyblade Wars will be come again!”
Aqua: “Shut up, I will not listen to your nonsense! You will return Ven’s heart!”
After the fight
Vanitas: “What? Finished already?”
Aqua: “Terra, Ven... give me strength.”
Vanitas: “No matter how many times you keep trying it will be the same.”
“What!?”
Mickey: “No! The power is going berserk.”
Secondly When does this take place? You said it wasn't an AU fic. Aqua never sees Vanitas without his helmet on. She is never beaten by him unless you count that sneak attack he did after she fought Braig a fight. She also never returns to the land of departure with Ven and has a conversation with him.
Thank you.
Edit: the fic in question.
http://www.fanfiction.net/s/5717266/1/Amber_and_Blue