Red itchy vagina can be due to a number of causes. First of all it can be due to allergic reaction or contact dermatitis to your undergarments, sanitary napkins or tampons, soaps etc.There is no change in the colour or smell of discharge in this case.
Secondly, It can be due to STD’s like Chlamydia or thrush but in such cases change in discharge is often present.
The soreness can be due to chafing. Chafing occurs when the skin becomes irritated by the friction caused by rubbing – either against other skin or against a foreign material. Wash the areas several times with fresh water. Do not use any cosmetic products at the sites. You can apply some calamine lotion at the rash as it will help in soothing the skin.Also wear cotton undergarments. Application of mild corticosteroid cream and oral antiinflammatories like ibuprofen/acetaminophen are also helpful. If the symptoms are severe then it will be best to consult a doctor.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.