Severe coastal erosion caused by Typhoon Kalmaegi exposed a centuries-old shipwreck in Vietnam, providing a narrow window to salvage what experts say could be a historically significant find.

Vietnam evacuated thousands of people from coastal areas as Typhoon Kalmaegi, already among the world's deadliest cyclones this year, closed in on the country's storm-battered central belt on Thursday.

Vietnam was rushing to ensure more than half a million people were out of harm's way on Tuesday, ahead of the arrival of a powerful typhoon set to dump more heavy rain on a central region devastated by weeks of intense weather.

Three men swept far from Vietnam's coast by Typhoon Kalmaegi have been plucked safely from the surf after drifting helplessly for more than 40 hours, a rescuer and the wife of one of the survivors said on Monday.

Vietnam is rethinking how it copes with floods after a year of relentless storms has collapsed hillsides and turned streets into rivers.