Variegation

Your Vinca plant is a variegated variety and has been bred to display the creamy-white markings on the foliage. Although variegation is technically a virus it is not harmful and is often a desirable feature bred in many plants. So nothing is wrong with your plant. Often the white portions that are heavily variegated can sunburn and if the new growth is all white and lacking green foliage (no chlorophyll present), as pictured with a couple of the stems on your plant, these stems will often not grow properly and eventually desiccate due to the lack of chlorophyll available for appropriate growth. Stems that are solid white and lacking chlorophyll can be removed (cut back) and easily corrected. Sometimes variegated plants revert and lose their variegation by essentially being given the ability to grow out of the virus. This can be avoided by trimming any stems with foliage that has reverted to solid green leaves. Variegation if usually very desirable and makes some plants more valuable.