The most simple explanation is that this particular "Ventus-Vanitas" during this opening symbolizes that unlike Terra-Xehanort, the possession was never fully complete.
Ventus was fighting against Vanitas on the inside but that couldn't be actually depicted in the opening, so they mashed both Vanitas- and Ventus-pieces together to show that this "hybrid"-being was always contested over.
The blue eyes and blond hair symbolize Ventus, the bodysuit, metal jaw piece and the black eyebrows symbolize Vanitas.
Seriously, you can see it plainly in Muke's signature that Ventus-Vanitas' eyebrows are black, something he also didn't had in BBS proper when Aqua fought him as her final boss.