How many users in user group M23-26 use a vivo device?
user group M23-26 refers to group = 'M23-26'; vivo device refers to phone_brand = 'vivo';
SELECT COUNT(T2.device_id) FROM gender_age AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.device_id = T2.device_id WHERE T1.`group` = 'M23-26' AND T2.phone_brand = 'vivo'