Vitiligo is a condition which is due to the loss of melanocytes or pigment cells in the skin. This leads to the skin appearing white or depigmented. Vitiligo can affect the skin and oral mucosa and the lip is an area that is not uncommonly affected.
Simple protective measure like using a lip balm with SPF may be helpful to protect the lip from the sun. Indeed, the loss of pigment can reduce the natural protection of the lips from the sun.
Topical steroids can be used to help in the re-pigmentation of the lips. There are some non-steroidal options like topical calcineurin inhibitors which can help. These are prescribed medications.
In some cases, localized phototherapy or skin/cellular grafting can be considered. This is particularly for treatment-resistant cases.
It is best to see your dermatologist to discuss these treatment options.