i'm not going to say its 100% sennas fault because vettel did take a wide line and turn in on him very much but lets be realistic here:
you're in the front and are taking a corner. do you expect somebody from what, 5 places?, to ram you off?
same goes for senna : you see the cars in front all on the right and you are confident you can brake later,albeit by taking a little bit of a wider exit:
do you expect that 5 places in front of you there is a championship candidate taking a line thats unfortunately completely contrary to yours?
the answer for both is no IMO. its 2 cars in a chaotic situation gunning for the racing line. i'm not going to blame either vettel (cause that would be BS) for the collision and not senna either.(only blame would be to behave yourself in a title decider but like i said,he probably saw a gap(like sennas do all the time when they see other cars) but no car on the outside turning in on him.note how vettel is already comitted to that line and that apex even before senna actually turns the car)
if anything that was a bit ambitious by senna and a bit of a strange choice for a line by vettel,racing incident nothign more