Raindrops on Vettel's camera showed it was raining as the incident happened. After the crash but before he ran off the tarmac, you can see the sheen of water on Vettel's tyres so it wasn't just damp, it was wet (at least in spots) off the racing line.
Here's what I saw happening watching in slo-mo:
Button braked for the corner and Vettel tried to check up. He never got unduly close to Button but he was close enough to upset his downforce. For a second or two before all hell broke loose, Vettel's car already was sliding a bit; his steering inputs weren't always producing a corresponding directional change. I wonder if maybe he was expecting flexi-wing downforce? From the instant it broke loose, Vettel was just a passenger.
Vettel did not steer left, it broke loose at the front and yawed left. He corrected right and the yaw reversed to the right. That the yaw reversed (and the car began to spin right) probably had more to do with the dynamics of a mid-engined car and his suddenly getting out of the throttle as from his steering input. While this was happening, Button was under braking but Vettel was not so Sebastien partially overtook Jenson. Almost instantly after the spin reversed, Vettel put in full opposite (left) lock but the car continued spinning right until he'd speared Button.
More drivers have spun or slid off the circuit this weekend than haven't and I think the drive-thru was more the result of Vettel having turned in to his own teammate at Turkey than his mistake today. I think the stewards saw it as a pattern and jumped to a conclusion rather than fully investigating because I'm fair certain if they'd just examined Vettel's in-car view, they'd have labeled it a simple racing incident. Vettel definitely did not steer into Button, deliberately or otherwise.
if you cannot see it as an avoidable incident then what race where you watching. it was nothing to do with turkey all today did was affirm that vettel is far to aggressive and needs a much wiser head on those shoulders if he is to be taken as seriously as everyone wants to. the penalty was more than justified it was very clearly an avoidable incident and was lucky not to get a stop and go or more damage to his car.
and yes vettel did steer left to overtake it then hit the bumps at the edge of the breaking zone causing him to gain air and less grip he then over corrects bangs it straight into jensons car, he wasnt a passenger he could have simply gone wide when he hit the first bump.
Jenson Button has just said that it was bone dry at the corner of the incident with Vettel.
Except for all the raindrops on the camera. Quite obvious it was wet
they flying around the circuit very fast with it spitting ofc there is gona be spots on the camera lol, you cannot say he lost it because of the rain when plenty of others went around only minutes before overtaking / breaking off the racing line .
its a poor move, end of story.
go complain to the FIA if you're unhappy with the rulebook. 
