Write SQL query to solve given problem: Among the users who uses a vivo device, how many of them are female and under 30?. Keep the solution inside sql tag ```sql [SQL-Query] ```
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