Looks like powdery mildew. Petunia can also be affected, but don't know if by same strain on the vincas.

Possibly opportunistic since yellow leaves hint there might be some other stress. Could cut out as much of the PM affected parts and apply a high N feed. We find Hi-Yield Iron Plus Soil Acidifier pellets (or similar) often helps plants that look like that in our north Texas clay, but depends on your soil type and condition.

Don't know about chemical PM eradicants for small plants, but that could also be an option. We fortunately haven't had to mess with it much since leaving Houston.

Thanks for the info, I'll give it a try

It seems that annual vincas are very subject to fungal diseases. It used to be recommended that they not be planted until late May or early June after the weather got hot. Then a few years ago they came out with disease resistant varieties such at 'Cora' and 'Nirvana'. Read about them HERE.

I don't know either if the same fungus affects petunias, but the article suggest it might. Spraying as suggested may help, but you might also want to consider pulling out the affected vincas and beginning anew with resistant varieties in June. They grow fast.

Probably most varieties sold now are disease resistant, but even they could not overcome damp winter weather.