Mix cream and milk in a saucepan. Cut vanilla bean lengthwise, scrape the vanilla seeds out and stir seeds and pod into the milk mixture. Bring to a boil.
Beat egg yolks with the sugar in a bowl until creamy. Whisk the slightly-cooled milk mixture in a thin stream into the egg yolk mixture. Pour back into the pot, remove the vanilla pod and stir over low heat with a wooden spatula until the cream begins to thicken. Do not boil.
Pour mixture into a bowl set over an ice water bath and cool completely, stirring occasionally. Stir in yogurt.
Process in an ice cream maker.
(Optionally, pour mixture into a shallow metal tray and chill in the freezer for about 5 hours. Stir well frequently during the first hour of chilling and occasionally thereafter so that no large crystals form).
Scoop into balls and serve in bowls.