Well, Ventus was around when Sora was still on Destiny Islands, as a kid. While Roxas, who is Sora's Nobody, showed up after Sora went to sleep...
Ventus fell into sleep, his heart was broken coz xehanort tried to forge the X-Blade and it failed and Ventus loses his heart. So at the ending of BBS His lost heart wanders and eventually finds child sora. In order to heal Ventus' broken hearts, Sora allowed him to merge his heart and share it with Sora'a... So a part of Ventus lives in Sora... By the end of KH1 When Sora stabs himself and losess his heart, he also released Ventus' heart which created Sora's nobody; Roxas, he had the same physical apperance as Ventus... But instead of just going into darkness, Vens heart helped shape Sora's Nobody, Roxas. This was because Sora had Ventus' heart, when his nobody came to be, it took the appearance of Ventus. So no Ventus is not Roxas, they are two seperate beings, but Roxas may not necesserily be just a nobody... He feels emotions..! Also Roxas might not be Soras nobody... Axel thinks Roxas isn't Sora's Nobody is in his journal which I quote, "I just don't see it." Meaning that Nobodies who look human are supposed to match their original selves by look. And Roxas resembles Ven. Roxas is my most favourite character in the KH Series..!
Nope i havent, its a 3ds game and i dont have a 3ds... and i didnt read the plot as well... Why do you ask?
You may want to watch this scene then. Spoiler http://www.youtube.com/watch?v=ARCX-Kv1TIU Skip to 2:43. Also pay attention to Xemnas's dialogue.
