Having read the question How to get the flavors to meld when I add fruit and sugar to plain yogurt?, I still have a question. I enjoy vanilla-flavored greek yogurt. I know pure vanilla extract is a very strong flavoring. How much vanilla should I add to a cup of plain greek yogurt? How much Splenda to the same amount of yogurt? 
This is a matter of taste.  You should experiment and see what you prefer.  Based on these blog entries chosen because they come early in search results for "homemade vanilla yogurt"   Blissfully Content Life Vintage Mixer   you might want to start your experimentation with the following ratios:   3 tablespoons of vanilla per gallon of yogurt 3/4 cup sugar equivalent per gallon of yogurt   On a per cup basis, this works out to about 1/2 teaspoon of vanilla, and 2 1/4 teaspoons of sugar or equivalent. 