China denied the allegation on Wednesday and said the fishermen had been detained for illegal fishing.
It was the latest confrontation boosting tensions in a sea dispute between the Communist neighbors.
Vietnam's Foreign Ministry said in a statement posted on its website late Tuesday that a Vietnamese boat which was fishing near the Paracel islands on August 15 was intercepted by two Chinese speedboats.
It said the Chinese vandalized the fishing boat, confiscated property and beat the fishermen. The Paracels are occupied by China but also claimed by Vietnam.
Vietnam demanded that China severely punish those involved in the incident and compensate the fishermen, Foreign Ministry spokesman Le Hai Binh said in the statement.
The delay in lodging the demand was because it took time to investigate the incident. Vietnam lodged a protest with the Chinese Embassy in Hanoi.
Relations between Hanoi and Beijing plunged to their lowest point in years following the deployment of a Chinese deep-sea oil rig near the Paracels in May.
The oil rig deployment sparked anti-China protests and riots in some parts of Vietnam, resulting in the deaths of four Chinese and injuries to hundreds more. Protesters also set dozens of factories on fire and damaged hundreds of others they believed were connected with China.
China removed the oil rig in July after it said it had completed its work, and since then the two sides have been trying to mend relations.
