i say bits and pieces. Roxas had Ventus's heart, so in turn Ventus's heart felt the agony of losing Xion and felt the happiness of see salt ice cream (not many happy things happened, but you get the point). So ventus's heart in turn did not directly see what was going on to have those memories, but what Roxas felt, Ventus will remember and feel when he wakes up. Im almost certain that when Ventus wakes up, there will be a tear in his eye, and he will scream out " Xion! Axel!" and start sobbing or something. He might also be like" So the boy Lea who i met in Radient Garden turned out to be one of my best friends.... Who knew!!!" then a whole conversation with axel as such:
Axel: What! Roxas is that you!
Ventus: Wait are you that Lea kid.... the one that i met in Radient Garden?
Axel: Wait so Roxas was actually Ventus's this whole time... No wonder Xigbar was giving him the "evil eye". So that's what he meant. I wonder if he still remembers the times we had back in the organization? :unsure:
Ventus: So Lea, when are we gonna go back on top of the tower and eat sea salt ice cream again?
anyway, i was getting off topic, but yeah that's what i think.