i believe that vanitas is actually a good guy. at first he is with MX. but then later MX says to terra that he needs to go destroy vanitas. and we all know MX is a bad guy. nomura saiid we should pay attention to who Vanitas really is. so im guessing vanitas was pretending to be friends with MX. and MX was training ventus for something but vanitas wanted to train him for something else. so he helped train ven, but for another reason. then when MX did something wrong im guessing thats the time when vanitas double crossed him. also i think vanitas was training ven to beat MX. terra just beats him to it.