In a sauce pan, bring vinegar/juice to a boil. Reduce slightly.
Pour only a tsp of the butter/vinegar mixture into the cold egg/water. Whisk.
Slowly incorporate the rest of the butter sauce.
Return the mixed sauce to the pan and VERY CAREFULLY let the sauce re-heat. TIP: stop whisking for a second and watch the edges of the pan. As SOON as you see bubbling around the OUTSIDE of the pan pull it off the heat and whisk. This should heat and cook the eggs (sauce will thicken) but should not scramble them.