How many events in total have happened on all the vivo devices in the year 2016?
vivo devices refers to phone_brand = 'vivo'; in the year 2016 refers to year(timestamp) = 2016;
SELECT COUNT(T1.event_id) FROM events AS T1 INNER JOIN phone_brand_device_model2 AS T2 ON T1.event_id = T2.device_id WHERE STRFTIME('%Y', T1.timestamp) = '2016' AND T2.phone_brand = 'vivo'