Vitamin C doesn't actually influence the white blood cell count. However, vit c deficiency, just as many other deficiencies, could weaken your immune reaction.
I am taking magnesium, zinc, pottassium, b6, b12, D and iron (with vit C) and all of this works just fine with aza. As a matter of fact getting rid of deficiencies is key to stay in remission... As least as I see it. Despite all those supplements my white blood cell count has been between 4300 and 5200 per mcL for the las year or so in all tests (this is where it should be with aza, at the lower end of the normal range which is 4 to 11,000 per mcL).