Question: Among the users who uses a vivo device, how many of them are female and under 30?
SQL: SELECT COUNT(T1.device_id) FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T1.gender = 'F' AND T2.phone_brand = 'vivo' AND T1.age < 30