Dear Welcome to HealthcareMagic!
There are a huge number of hypopigminted as well as hyperpigmented birth marks. Many of these are usually not apparent at the time of birth & are usually more or less permanent, growing in proportion to baby's size. Your baby probably has a 'naevus achromicus' which is a fairly common hypopigmented naevus; it is not a vitiligo
lesion. There is not much to worry. Please get a confirmation by showing it to a dermatologist