If you are trying to get the flavor as close as possible to real key lime pie, I think vanilla greek yogurt would help mimic the flavor much better than using plain, but that is just my opinion. Maybe you can try a lower sugar vanilla like stonyfield farms? It has 12g of sugar vs 6g in the plain. If you must use plain, I would add vanilla & some raw sugar or agave. Personally, I think stevia has too strong of a taste for this flavor combo.
i would guess that fruit on the bottom style would be the most effective. in other words, start with at least a cup or so of lime juice, add your sweetener, heat it up, add a thickener (cornstarch, arrowroot, agar agar), cool it and then add it to the yogurt. greek yogurt already has that concentrated, tangy flavor, so you're going to need to concentrate the flavor of what you are using enough to compete with the greek yogurt.
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