Depending on when you planted the Vinca, they may be progressing through symptoms caused by cold weather. Annual Flowering Vinca (Catharanthus roseus) is a warm-weather plant, not growing well in cool weather and cold soils. They should be planted no earlier than mid-May, though this year we stayed cooler for longer than usual. Like tomatoes, when planted too early, they are unable to adequately nourish themselves in cool soil. Adding fertilizer will not help in this case, as it is a physiological issue the plant cannot overcome until temperatures warm. Since they seem to grow out of it as the weather warms, this is a likely cause.

If the plants were planted only recently (in the past four weeks), then they may have poor root systems or are suffering root dieback from staying too wet. Vinca like to dry out somewhat (not completely) between waterings, rather than staying constantly moist. Compromised roots will also not be able to absorb nutrients sufficiently, leading to signs of deficiency in the foliage. Adding fertilizer in this case will similarly not benefit the plants, because they need to regrow roots first.

Some nutrients can be shunted into old leaves after the plant absorbs them while others cannot. This is why picking off the yellow leaves is probably the best approach, as they will not green-up over time, even if new growth looks good and evenly green.

We do not see signs of pests or disease, though if any plants wilt in the future, check the soil moisture before assuming they need water. One or more of the diseases that Vinca can contract will cause wilting, even if the soil isn't dry. In such a case, watering will only worsen the problem. The fertilizer you've already used won't hurt (and may even be needed sooner or later), but avoid applying it more often (or more concentrated) than the package recommends.

Miri

Thank you, Miri!  I appreciate the time you took to explain how these plants work!

You're quite welcome!