I would assume that you would be declared fit on the assumption that there are no other areas of medical concern.
It could be that the Senior Medical Officer (Service Entry) might take a look at the paperwork to give the go-ahead, as JSP 950 says that 'vitiligo not associated with any other autoimmune disorder may be graded FIT' and he might want to check with your GP that your case matches these criteria.
I would begin your application now, mention the vitiligo on the form and then discuss this with the medically qualified staff involved in your recruitment process. Start the ball rolling today!
By the way, I don't know a great deal about vitiligo myself, but it may be that you will be given special advice about protecting your skin if you are deployed somewhere where the sun is intense.
I suddenly 'got' slight Vitiligo when I was about 21. It manifested as 3 or 4 inch / 8 to 12cm patches on the inside of my elbows and around my armpits both sides. Maybe some other small bits - can't remember. White against my skin in summer, red against my skin in winter.
I was a sun-god and had just been on a long and sunny deployment and initially thought it was just some sort of heat rash. It was diagnosed as Vitiligo. Didn't cause me any problem apart from slight embarrassment.
After about 5 years, one summer it just disappeared - no white patches.