I have concluded that it is best not to put vinegar and salt in the water when you poach eggs: they soften the whites, which fail to cohere. But their softening effect is ideal for scrambled eggs. To four eggs, I added 1tsp of white wine vinegar, which we could not taste when the eggs were cooked. They were deliciously soft and creamy.
Melt 1tbsp butter in a non-stick pan. Beat the eggs lightly (to encourage them to set) with the salt and vinegar, pour into the pan, and cook over a low heat, stirring regularly. Meanwhile, keep another tbsp of butter, cut into pieces, to hand.
The trick is to judge the moment when to take the pan off the heat. The eggs should be approaching the perfect state, in which they are set but still creamy, but not quite there - they will carry on cooking in the hot pan. Stir in the remaining butter, which will help to arrest the cooking process.