Heat milk in large heavy saucepan. Beat egg yolks with 1 cup sugar. Stir in about 1 cup hot milk. Quickly stir into remaining hot milk and cook, stirring constantly, over very low heat until mixture thickens and coats metal spoon. Remove from heat and add vanilla. Chill. Stir in undiluted orange concentrate. Pour into punch bowl. Beat egg whites until foamy. Add remaining 1/2 cup sugar, 2 tablespoons at a time, and continue beating until mixture stands in stiff peaks. Float heaping tablespoons of meringue on top of eggnog. Garnish with orange sections.