Time to allow my input into this debate, and I'm fairly neutral (granted I'm Australian, but I'm an Indian supporter and have been hoping for an England win this series). In my opinion its pretty silly to aim any criticism at Vaughan for taking the light. All the criticism in the world can be aimed at him for the substitution issue which was raised last test in regards to being not in the spirit of cricket, but as captain of England who has the opportunity to win the Ashes and topple Australia, a feat not achieved for something like 15 years, I see him doing nothing that most captains would do. And even if some captains wouldn't have (I saw someone here mention Waugh and Taylor as examples) that doesn't mean Vaughan's decision was 'against the spirit of the game'. If that's the case, everytime someone goes off for the light when it is in their team's best interests are not playing in the spirit of the game. If that is someone's viewpoint, well then I think they're flat out wrong.
The criticism aimed at the umpires I can understand, but at the same time I believe its way overblown. What the problems are is the clarity of the issue. This occurred in South Africa back in December/January, and various English members were flagrantly angered (granted Vaughan didn't get a chance to bowl his spinners) but no one blamed Smith to my knowledge. They were rather irritated by the current laws. I think we need a flat out obvious ruling stating if the umpires believe playing on with the current light conditions is physically dangerous, they can offer it to the batsman. The question is, is it only about safety or rather "fair conditions?" I think its the latter, and thus a clear law should be written out exemplifying that. Whether Shane Warne can hurt someone or not, if the law's aim is to allow the batsman every opportunity to be able to see the ball clearly whilst batting, then it should say it straight up.
Now the question is, was the light that bad that with spinners bowling the batsmen couldn't see the ball properly. If that's the case, the right decision was made IMO (assuming that's what the law aims to do, provide fair conditions and not just safety).