In rainy weather, vinca can be infected by phomopsis, a fungal disease that causes the shoot tips to wilt, turn brown, and die back to the soil’s surface. Affected stems may turn black. Plants may die out in patches. To get your problem under control, prune out and destroy infected plants so the disease will be less likely to spread to healthy plants.
During dry weather this summer or fall, you may also want to thin out the healthy plants to improve air circulation and promote faster drying of the foliage. (Also consider thinning out branches of surrounding trees and shrubs to improve airflow.) These cultural steps may be enough to get the disease under control. You could also use a preventive fungicide spray to keep the disease from reoccurring.