If it is disease, and you know someone willing to move many wheelbarrows of soil, you might have an opportunity to keep it from spreading to the rest of your bed. Excavate those spots, replace with known-clean soil, and use the questionable soil to improve some bed that will never be used to grow Vincas. Like shrubbery or vegetables.
Then, plant something as different as possible from Vincas in and around those spots for several years, to reduce the number of spores in the soil.
But it surprises me that any soil disease did not spread rapidly. Do you follow "no-till" practices?
I'll mention another possibility only because I am always obsessed with drainage. When you remove the plants, do you inspect the roots and the soil they werein? Big fat healthy-looking roots? Just a few skinny threads? "Open" soil with air spaces, or glueey clay?
If the red clay soil was not well mixed with amendments, there might be "clayey spots" that hold water, especially if the slow-draining zone was shaped like a cup or bowl.
But you could rule that out with a little spade work or forking around those spots, when you replace the plants next.
If you don't till and mix your soil, maybe there are clumps with bad drainage right in those spots. Or big rocks near the surface.
One thing you could do to rule out some possibilities would be to put a slightly raised bed in those spots, so the plants have some root zone above grade, assuring at least some aerated root zone near the surface. Maybe just mound the soil up before re-panting, or use some kind of edging to support 3-6" of raised soil.
Here is another possibility, as bad as soil disease.
Could someone before you have dumped oil or herbicides or other unhealthy chemicals in just a few spots?
If you have a crazy neighbor on the warpath, could they have sprayed herbicide in just a few spots?
Could you have gotten a batch of soil amendments with (for example) very saline clumps, or too-strong fertilizer, or herbicide residue? Manure with salt and/or herbicides?
