The Vienna Convention is indeed a cornerstone of international relations and I would also be very upset to see it violated. But fundamentally it's about the rights of Westphalian states, not human rights. I think it's entirely fair for Amnesty International to say that they're only interested in securing human rights.
You cannot have human rights without peaceful international relations. You cannot have peaceful international relations without the Vienna Convention.
If Amnesty cannot see that, they're unworthy of their fame. Unfortunately, I suspect they do see that, and nevertheless they're not acting. Which is even more damning.
When Assange is on trial in the US and in jeopardy of being executed will you say that is not a case of "human rights"?
Hint: the women in Sweden won't be the point of inquiry then. It will be his actions on the internet, for which he most definitely should not face death.