Hello Alice,

If you look closely, the discoloration is not irregular sized and shaped like you would expect from a fungal disease. The discolored and dried areas on the leaves are made up of relatively equal-sized circular areas. This is consistent with a piercing/sucking insect feeding on the juices of the leaf.

You can cut off the damaged foliage and monitor the plants for continued damage. If you see more spots developing, turn the leaves over and look for a pest on the back side of the leaf. If you don’t see anything during the day, go out at night and inspect the leaves with a flashlight. If you see more damage and you see the pest, take a picture and send it with this question.

Aphids are generally the most likely suspects on vinca, but these dead areas seem to be too large for aphids.

Hope this helps.