Rosberg should have been a non-issue. He had plenty of room to the left. Of course hind sight is 20/20. And it's probably considerably worse from the cockpit of an F-1. I backed it up and went frame by frame. Out of all involved, imo, Vettel was the least at fault.
http://www.formula1.com/en/video.htm
At around 0:11 to 0:12 Verstappen moves well towards the center of the track leaving a huge opening for Vettel. By 0:14 Vettel is well beside the Red Bull. At 0:15 Verstappen contacts Vettel's Rear left wheel. At 0:14 Roseberg realizes he's about to lose second to Vettel and attempts to drop down in front of the whole field appearing to crowd Verstappen which causes the contact with Vettel.
The reason I make a deal of this is, Verstappen was very vocal from the cockpit. Maybe trying to shift blame for his own dumbass move? "Nature abhors a vacume." So seems to be the same with F-1. You can't leave a gap like that on a start and not expect someone to fill it.
This link to the start evidently doesn't work. It was the smoothest video of the ones I looked at. So if you want to see frame by frame you'll have to go online to find it. Sorry.