Huynh Truong Ca had posted online articles and livestreamed videos calling for change in one-party communist Vietnam.
The Brotherhood for Democracy member’s sentence was upheld in a heavily guarded, three-hour trial.
Police say that organizers did not get advance permission to hold the event.
The document sent to national leaders, the UN, and foreign diplomats urges release of political prisoners and guarantees for the rights of assembly and a free press.
Tran Thi Nga had been refused permission for a visit in September as punishment for 'breaking prison rules.'
Family members say the activists endure routine abuse and suffer from health problems in detention.
Nguyen Van Trang and Le Van Thuong face charges under Article 109 for their pro-democracy efforts in the one-party state.
Nguyen Trung Ton endures daily coercion to confess, but maintains innocence.