How to stop vaginal pre menstrual itching?

First: get diagnosis. Part of the problem is that itching is a symptom of another problem. You could be itching from hormone changes, infection (yes, even if it is every month), or skin disease like lichen sclerosus. The treatment for these is all different. You should see a gynecologist and get an exam at the very least to determine what is wrong first. Try to go when you are at your itchiest.