In a bowl, mix the yogurt, key lime juice, Splenda, and vanilla extract until smooth and well combined.
Taste and adjust the sweetness or lime juice to your liking.
Put the graham cracker square in a zip-up baggie. Use a meat mallet or rolling pin to turn the cracker into crumbs.
When you're ready to eat the yogurt, sprinkle the graham cracker crumbs over the top.