Among the devices with event no.2 happening, how many of them are vivo devices?
event no. refers to event_id; event_id = 2; vivo devices refers to phone_brand = 'vivo';
SELECT COUNT(T1.device_id) FROM phone_brand_device_model2 AS T1 INNER JOIN events AS T2 ON T2.device_id = T1.device_id WHERE T1.phone_brand = 'vivo' AND T2.event_id = 2