Shingles is the result of the same virus that causes chicken pox. Once you have had chicken pox, the virus remains dormant for years and has resurfaced as shingles. So in that case, the infection actually occurred when you got the chicken pox which for most people is long before they ever had any cancer treatment.
I don't think vitamin c or any herb or supplement is going to increase your white blood cell count. That is what makes us more vulnerable to infections during chemo because it destroys white blood cells. Your oncologist should be continuing to run periodic blood panels until your WBC is back up a normal range.