Vietnamese state media said a patrol boat has exchanged water cannon fire with Chinese vessels.
The incident took place near the controversial CNOOC oil rig recently moved by China into disputed waters.
Two journalists for the Tuoi Tre newspaper reported they witnessed the exchange in the South China Sea on Monday.
Both countries have accused the other of intentionally ramming their ships.
The journalists reported Vietnamese vessels are displaying banners in Chinese urging Beijing to remove the rig.