Images in state media reports show the obscene words scribbled twice over maps of the contested South China Sea.
The Chinese consulate in Ho Chi Minh City said in a statement yesterday it strongly condemned the "shameless and cowardly" act and asked Vietnamese officials to investigate and punish the agent.
China and Vietnam have had long-running territorial disputes in the South China Sea. Tensions spiked in 2014 after
China parked an oil rig near Vietnam's central coast.
China issued new passports starting 2012 with revised maps to include the "nine-dash line," demarcating Beijing's claim to nearly the entire South China Sea.