i think kubica wanted the race win, that's why he didn't wait for a better corner. he wanted to get past vettel asap.

and from vettel himself:
explained Vettel. “At the time I turned in I was ahead, but I couldn't keep up speed in the corner and Robert was on a harder tyre, so was much quicker. At the time we collided he was in front, but I had no where to go, I couldn't stop the car, or turn to the right and my tyres were gone."
“Maybe I should have said let him go and bring third back home, but that's life. I tried to defend and, up to the mid-corner, I had reason, but then I had no grip to avoid a collision. I'm sorry to the team and also to Robert, as it didn't just mean the end of my race, but also his.”
The Official Formula 1 Website