Heat a small sauce pan half full with water until it is simmering lightly.
In a small metal salad bowl, whisk the water, soy, and egg yolk well. Now place the small bowl over the pot of simmering water and whisk continuously for a few minutes until the mixture starts to look a little creamy, thickens and warms up a bit, about 4-5 minutes.
Make sure that the water isn't at a rolling boil, or it can cook the egg in the mixture and make it lumpy. (If you cook it too much and the egg coagulates like scrambled eggs, don't fret, you can puree the mixture with a hand blender and strain it. or just start over, its not that tough to do.)
When the sauce has thickened enough to coat the back of a spoon, add the vinegar to taste, until it is as sharp as you like, then set aside and reserve. Essentially you have made a creamy egg based salad dressing, something of a cousin to mayonnaise.