Is what the questioner heard in a dream, from a voice commanding him to perform ghusl and drink the water from his wife's ghusl for healing, considered a righteous vision, and is it permissible to rely on it for the treatment of others, or is it specific to him?

You have done well by relying on Allah and by performing ruqyah (Islamic incantation). Judgments are not derived from dreams, and there is no legal or rational evidence for benefiting from drinking water with which a person has washed. It is better to recite ruqyah over water and drink it. Ibn Al-Qayyim experienced this by reciting Al-Fatihah over Zamzam water, and Imam Ahmad did so by reciting the Qur’an over water, drinking it, and washing with it.