My neighbor is having a problem with his flowering annual vinca this fall. Five out of 10 vinca plants began curling of leaves; it appears the plant is wilting from lack of water (which is not the case), and eventually the leaves fall off leaving only the stem. No sign of holes or chewing of leaves s apparent. The other plants in this bed are healthy and doing well. Soil is perfect and he doesn't use pesticides or herbicides. The plants were fine all summer and this just recently happened in the last 2-3 weeks.
First question: Are you certain that it's vinca?
What you describe is typical of downy mildew on impatiens. We are hearing about it right now. It is why they generally aren't being sold anymore.
If you are not sure, send us a photo of the plants (inc. a flower), which you can attach right to this reply by clicking on 'browse' below.
If you're sure it's vinca, it sounds like the wilt disease phytophthera. This pathogen is always around, but when it is a wet season it can become a problem, and vinca is one of the more susceptible plants. Others include petunias, snapdragons, sedum and Calibrachoa (million bells).
Here is our page about wilt diseases: http://extension.umd.edu/hgic/wilt-diseases-flowers