Vaginitis is one of the main causes of a red, itchy vagina. There are a number of reasons that a woman may experience vaginitis, and I'll go into a little more detail about these below.
Many women will suffer from thrush at some point in their lives. Basically, it's a yeast infection that occurs when too much Candida becomes present in the vagina. Thrush can be quite irritating, but can be treated with over-the-counter medicines in most instances, and is not an STI.
Chlamydia
Chlamydia is a very common STI, and brings with it some nasty symptoms, including pain during urination, discharge, and irritation. Although chlamydia is a sexually transmitted disease, it can be treated with antibiotics relatively easily.
Bacterial Vaginosis
This is less likely to cause the vagina to become red or itchy, but has been known to. Other symptoms of bacterial vaginosis include a fishy smell and discharge. It occurs when there is an unbalanced amount of bacteria within the vagina. Once again, antibiotics can be used to treat the condition.
Soap
Another common instigator of vaginal irritation is soap. Your body can react badly to strong soaps, especially if they manage to enter the vagina and mess around with the bacterial balance. Make sure you're using a sensitive soap, or alternatively, simply wash thoroughly with only warm water.