Blend the mango and yogurt together (and the vanilla if using). If it's too thick to drink with a straw, add some milk to thin it out. A ripe and juicy mango combined with thin yogurt is often all you need, but if your yogurt is thicker you may need to thin it out. Be warned: if you make this for children, they will request it over and over.
Notes
You can substitute frozen mango, you may just need to add a bit of water or milk so it blends enough. Add a splash of vanilla for extra flavor if you have it.