Put the egg yolks in a saucepan. Add the water, rum, sugar, vanilla sugar to the egg yolks and stir to a mixture. Add a few dashes of lemon juice and vanilla flavor. Heat slowly in a water bath, stirring constantly. When the liquid thickens a bit, the punch is ready.
Put the punch through a filter into a punch (you won`t be able to completely avoid the egg clogging up slightly in the punch, so I prefer to filter it again). When the punch has cooled down, sieve the punch again.