Is it necessary to fulfill a vow to name a daughter "Juwayriyyah" if Allah blesses one with a daughter, after having made such a vow and before knowing the gender of the fetus?

Naming a newborn with a specific permissible name is allowed. If a person vows to name his child a certain name, then his vow is a permissible vow. The one who vowed has the choice between fulfilling his vow or atoning for it like an expiation for an oath. The Permanent Committee has issued a fatwa (definition) permitting the naming of the newborn with the vowed name, or changing it with the obligation of an expiation for an oath. So, if you are blessed with a daughter, you may name her "Juwayriyah" in fulfillment of your vow, or name her with another name along with the necessity of an expiation for an oath.