What is your zone? That might give a clue what happened. It's been a wild ride weather-wise where I am, quite cold and damp for May. Vincas HATE cold soil, it will do them in if it lasts too long. Your plants look dried up on the top, I wonder if the roots just couldn't uptake water fast enough to keep up with leaf transpiration if the soil was too cold so the tops burned up on you. Over-fertilization might be a possibility if you doused them with fertilizer right after planting -- full-strength rapid-acting fertilizer is particulary harsh on newly planted plants.