Is it obligatory to offer two sheep for the aqiqah of a female, based on a vow, or should it be reduced to one sheep in accordance with the Prophetic Sunnah?

Your statement: "I will sacrifice two sheep for aqiqah (definition)" is not considered a vow unless you intended it as such. A vow requires a specific utterance or phrasing that conveys commitment. If you did intend it as a vow, then you are obligated to fulfill it. What has been reported from the Prophet, peace and blessings be upon him, regarding aqiqah, is by way of recommendation, not obligation. Scholars have differed on the number of aqiqah sacrifices. The Malikis held that one sheep is sufficient for each newborn, whether male or female, and there is no harm in sacrificing two sheep.