Question: Among all the users who use a vivo device, what is the percentage of the users in the M23-26 user group?
SQL: SELECT SUM(IIF(T1.`group` = 'M23-26', 1.0, 0)) / COUNT(T1.device_id) 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'