I usually recommend waiting until chicks are a week old before starting treats, and I personally wait even longer. Chicks don't need to have "treats" at all.
Vanilla yogurt has a fair amount of sugar, plain would be much better.
I was told that it's very important to give yogurt the first day that you bring them home so that they build up good bacteria in their intestines because it will help prevent pasty but. Last year we had 50 meaties and we lost 9 to pasty butt because it was our first year and we didn't know to feed them yogurt. This year we are almost to 24 hours with no casualties. If we can make it to 1 week with no losses I would say that I will be sold on yogurt for all my babies.
I have had my babies almost three weeks and I did not give them yogurt. Just there crumbles and water then around the 2cnd week we tried crickets and mealys then we added chick grit they have doubled in size and are very happy.