Thanks for your query.Based on the facts that you have posted it appears that you have
infection of the foreskin .Vitiligo
generally affects the skin that is exposed to sun light.and I have not seen any patent of Vitiligo of the foreskin in my my practice for last 40 years.
The discolouration of foreskin and cracking on erection is due to infection of the foreskin called as Posthatitis.
Please consult qualified Urologist
for clinical examination to confirm the diagnosis.
Use of oral antibiotics
with topical antibiotic
ointment like Neosporin
for a week will help to cure it.
If retraction of the foreskin is painful you may need to get circulcised.