Sometimes Vinca can suffer from bacterial diseases such as botrytis that are practically undetectable when they're in packs at the garden center. Weakened, they stress easily when transplanted into the garden, and the yellowing leaves are a common symptom. However, if the weather is warm and dry, the plants can often grow right through it and hardly show any symptoms. (I'm guessing you've got warm weather there in Tennessee.) On the other hand, if the weather is cool and rainy, yellowing leaves on a disease-weakened Vinca is almost guaranteed. 

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If you think a disease and weather combination might be your problem, hope for a warm, drier spell. Water only when the soil is dry at least an inch below the mulch. Some of the yellowing plants will continue to decline and die; you may have to replace them. The only other possibility for yellowing leaves would be fertilizer burn, but that's unlikely. You'd notice the yellowing and then browning at the leaf edges and tips first, not just an even yellowing over the whole leaf.