Vinca

[Clayjacks]

With plants like vinca, I practice "when in doubt, tear it out." Just keep an eye on the rest -- *if* it's a phytopthera-like disease, it can spread by splashing water (hose or rain). *If* it's a root disease like Pythium, it can spread through standing water or over-saturated soils. In my opinion, vinca isn't worth treating with fungicides unless it's a huge landscape feature, or you're growing commercially. They're pretty tough if the conditions are right (pH around 6ish, low-to-moderate fertility, plenty of direct sunlight and well drained soil.)
For one plant, I'd just rip it out and watch.