The South Pacific island nation of Vanuatu and the Soviet Union have signed a one-year fishing agreement that allows Soviet vessels to use Vanuatu's ports. The agreement has been criticized by the United States, Australia and New Zealand on grounds it will give Moscow a foothold in the South Pacific. Vanuatu, west of Fiji and 1,400 miles northeast of Australia, will permit Soviet fishing boats to use its ports to resupply or make repairs. The pact stipulates that a maximum of eight Soviet vessels can fish in the country's 200-mile offshore economic zone--for an annual fee of $1.5 million. It prohibits fishing inside a 12-mile territorial zone.