Praise be to Allah.
There is nothing wrong with using vaginal dilators to treat vaginismus, not for the purpose of unlawful pleasure. It is permissible to do that with the help of the husband, and that should be done if the woman fears masturbating, because masturbation by the husband’s hand is permissible.
Assuming that using dilators will lead to arousal or masturbation, and the aim of that is medical treatment, there is nothing wrong with that, in order to enable the wife to have intercourse and fulfil the objective of marriage, which is to keep both spouses chaste, provide pleasure and produce children. A number of jurists have permitted masturbation in the case of necessity, by way of committing the lesser of two evils.
Al-Mirdawi said in al-Insaf (10/252): We learn two things, one of which is that masturbation is not permissible except in the case of necessity…
The second is that the ruling for women concerning this matter is the same as the ruling for men. End quote.
And Allah knows best.