There's a difference between believing in one's own moral superiority and being a braggart; Aqua was in many ways self-righteous throughout BBS but she never gave any indication that she saw herself as stronger or more capable than her friends, rather the opposite, her whole arc was about coming to accept her own strength and take on the mantle of Keyblade Master without equivocation. Again, we have to consider what is at stake here: this is not a 1v1 showdown where, if Aqua loses, it's her own heart on the line. She has been struggling to reunite with Ven and fulfill her promise to him for so long now that it simply is not believable that she would do anything to endanger that promise. She's not fighting for herself here, so it makes no sense that she would play it out as if this is "her" battle to win or lose. There's no reasonable explanation behind this except that they needed to set up Aqua to get rescued by Ven, which is exactly what happens.
Speaking of which
So she has the ability to create extremely strong, protective barriers which require incredible power to break through, buuut she just elects to take the full force of Vanitas's blast directly. Lol, OK. At least now we know that Aqua's weakness is apparently any and all projectiles; sure hope she doesn't have to face Xigbar over the course of this game or she's a goner for sure.