What's your question? Ask

I wish I knew where you live and what type of vinca this is. If it is Madagascar periwinkle, Catharanthus roseus, then it may be shutting down for the winter. It is only hardy to zones 10-11. The Missouri Botanical Garden suggests taking cuttings and bringing them indoors for the winter. http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=a569

This is, certainly, the start of a fungal infection. The soil should be let to dry out down to at least two inches. Leaving the soil consistently moist will often lead to infection. It will also appreciate some shade during the day if it does not get any.

Also, it is important that the container has plenty of drain holes. If there is not enough drainage, it can appear dry on top but water logged down further, which will cause infection.

At this point, I would recommend dolomitic lime and wettable sulfur. Sulfur is a good antimicrobial, and dolomitic lime will repair the pH from the action of the sulfur.

This will, usually, kill off an infection as long as it is not something incurable such as Mosaic Virus.

This article will give you more information on the care of these plants: https://www.gardeningknowhow.com/ornamental/groundcover/periwinkle/growing-periwinkle.htm

Yellowing stems is a sign of watering issues. Make sure the soil is well draining and that they are not sitting in soggy soil.

It is best to let them proceed naturally from here. Since they are annuals. Once they die off, you can begin to plant winter crops.

This article will give you a good list of things that look good year-round: https://www.gardeningknowhow.com/garden-how-to/gardening-by-zone/zone-7/zone-7-year-round-plants.htm

I would also make sure to amend the soil when you know it will be wet for long periods of time. DOLOMITIC LIME, and WETTABLE SULFUR will be best suited for wet areas. This will prevent/kill any bad microbes in the soil, as well as condition it for planting, providing trace minerals, and extra protection.

Do this once per year to prevent infections of soil.

The best way will be to sink a physical barrier about a foot into the ground around the area that you wish to keep it in. This will help keep the roots from spreading. Keeping the tops cut before flowering will keep it from sending seed outside of that area.

This article will offer more information on ways to control aggressive plants:

https://www.gardeningknowhow.com/plant-problems/weeds/aggressive-garden-plants.htm

That is Vinca, and it will not go without a serious fight. Here is an article that will help:

https://www.gardeningknowhow.com/ornamental/groundcover/periwinkle/periwinkle-control-methods.htm