Write SQL query to solve given problem: What is the ratio of female users to male users who uses a vivo device?. Keep the solution inside sql tag ```sql [SQL-Query] ```
SELECT SUM(IIF(T1.gender = 'M', 1, 0)) / SUM(IIF(T1.gender = 'F', 1, 0)) AS per FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T2.phone_brand = 'vivo'