A DISEASE PROBLEM WITH ANNUAL VINCA
Annual Vinca (Catharanthus) is generally a problem free plant. It requires little maintenance and I can not think of any insect issues. However, it can have some disease problems ranging from simple leaf spots to the harder to deal with ones such as Aster Yellows.
More common to them are the stem and root rots. One in particular is Phytophthora Stem Blight (Aerial Phytophthora). This can occur with or without the root rot.
Here dark brown to black lesions develop on the stem and everything above them dies. It is accompanied by the usual poor growth, stunted plants, and wilting, prior to the death of the plant. This comes from too much water or frequent watering. Another contributory factor is too much fertilizer as this makes for fast, weak growth which in turn makes the plant tissue more susceptible to disease.
Infected plants must be removed and destroyed. Practice proper water management, and avoid too much fertilizer. Spray with a fungicide like copper (organic) at least twice. This should protect any remaining plants and can be used as a preventative measure.