Answer: There are two different plants commonly called vinca and I think this is the cause of the confusion. One is Vinca minor, sometimes also called periwinkle. This one is an evergreen groundcover plant with blue flowers in the spring.
The other plant sometimes called vinca is grown as an annual flower. The botanical name for that plant is Catharanthus. Its flowers are pink or white. Here is a photo and description of it. You may need to cut and paste the complete url into your browser to make it work correctly.
It's really important to check botanical names when it comes to plants, because there are many instances where several plants can be called by the same common name. I hope this helps.
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