Question: Among all the devices with event no.2 happening, what is the percentage of the device being a vivo phone?
SQL: SELECT SUM(IIF(T2.phone_brand = 'vivo', 1, 0)) / COUNT(T1.device_id) AS per FROM events AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.event_id = T2.device_id WHERE T1.event_id = '2'