Question: To which user group do most of the users who uses a vivo device belong?
SQL: SELECT T.`group` FROM ( SELECT T2.`group`, COUNT(`group`) AS num FROM phone_brand_device_model2 AS T1 INNER JOIN gender_age AS T2 ON T2.device_id = T1.device_id WHERE T1.phone_brand = 'vivo' GROUP BY T2.`group` ) AS T ORDER BY T.num DESC LIMIT 1