How many women can't keep their vaginitis away?

Depends. Depends on the cause. There are some patients whom have issues with recurrent infectious (bacterial or yeast) vaginitis. This may require long term therapy. But not all vaginitis is from infections. There are other inflammatory or hormonally related vaginitis. Recommend evaluation with a gynecologist especially one that speciallizes in chronic vaginitis or vulvodynia.
Recurrent Vaginitis. "recurrent BV is generally defined as 3 or more episodes of BV per year, and one study found that 6-month recurrent rates were as high as 80%. Given the treatment challenges and the fact that BV is the most common vaginal infection for women of reproductive age, the disease causes great frustration for both patients and providers." this is a direct quote from dr. A tittleman of u penn.