If you mean Vinca minor  or periwinkle, the common evergreen groundcover then the yellow leaves most often mean your soil is not acid enough for this spring blooming plant to absorb iron from the soil. Lack of iron causes the foliage to turn yellow. Fertilize with a plant food made for acid lovers such as Osmocote ACR. Another cause of yellow leaves could be too much sun. Vinca minor prefers shade in the afternoon. One more thing to consider is drainage. If the site is in a low wet spot then the leaves could turn yellow if the soil is saturated with too much water. Keep growing, Marianne Binetti