All the viva's I've seen (i.e. watched people do them in my department) - you tend to have the viva, then you go out for 30 mins or so and then you get called back where they tell you what the result is. Of the 10 people I've known to have a viva, about 8 of them had to ask for clarification about what the result actually was, as they were completely unclear as to whether they had passed or failed because the examiners aren't often very straight talking I guess.
I suppose you do have the opporutnity to argue, but I'd imagine, they've already made up their mind before they even come to the viva. I suppose if you get asked for a major amendment and you think its completely stupid, then you can tell them why and they may re-consult eachother?