Police suspect the fishing operation was linked to illegal mid-sea gasoline trading given the length of time the vessels were at sea and the distance they were away from Vietnam.
The four captains of the vessels and the 16 crew – all of whom are Vietnamese – were arrested 35 nautical miles off the coast on Sunday.
They were charged with intruding into Thai waters.
The four captains were later accused of being human traffickers and 16 crew were identified as suspected victims.
Since 2015 fiscal year, the authorities have impounded 17 Vietnamese fishing trawlers and arrested 140 suspects, an informed source said.