Put the egg yolk in a medium nonreactive bowl. While continuously whisking, add the oil drop by drop, incorporating each drop before adding the next. As the mixture thickens and emulsifies, begin adding the remaining oil in a slow, steady stream, continuing to whisk vigorously, until you have added all of the oil. The mixture should be thick and glossy. Whisk in the vinegar, salt, and 2 teaspoons water until fully incorporated.