All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad is His slave and Messenger.
It is obligatory for a woman to wash the outer part of her vagina without the inner part when performing Istinjaa’ (cleaning oneself with water after answering the call of nature).
Istinjaa’ is achieved by thinking that you have most likely washed the place. Therefore, you have to pour water on the entire part (i.e. the outside part) until you feel that you have most likely cleaned it from impurity. Whichever way this is achieved is sufficient for you, but it is not sufficient to leave any part of the outer part of the vagina (that is stained with impurity) without washing it.
Allah knows best.