Three Americans floating on their pleasure yacht off the Vietnamese coast have apparently been seized by an armed vessel, requiring the Carter administration to approach Vietnam with a request for information about the incident.

American officials yesterday sought to play down the incident, presumably because of the precarous nature of U.S. diplomatic contacts with Vietnam. But message radioed by the Americans indicated their yacht was fired upon, then rammed by the unidentified hostile vessel approximately 35 miles south of Camu, the southern tip of Vietnam.

According to reports from ham radio operators who picked up the yacht's distress signals the three Americans radioed their names and passport numbers while reporting that "our vessel is being rammed" and finally saying that they had lowered their sails waiting to be boarded.

There was no Vietnamese response as of last night.

Two years ago, an American cargo vessel, the Mayaguez was captured by a Cambodian naval boat in the Gulf of Siam. The Ford administration responded by sending Marines to recapture the vessel in a battle in which scores of Americans and Cambodians were killed.

Yesterday, administration officials appeared determined to avoid overraction. "We don't know if in fact this vessel was attacked, by whom and under what circumstances," one official said. "We simply don't know." The official said there were no immediate plans to send any American ships to the area for a resuce effort.

The yacht's distress signals were picked up last Wednesday by a ham operator in Australia, the U.S. Coast Guard in Alaska and the Australian marine operations center at Canberra. According to the last transmission, the craft was believed to be closer than 12 miles to an island off the Vietnamese coast.

Vietnam has imposed a 200-mile off-shore limite which was understood here as a claim on economic resources within wuch areas. But U.S. officlas believe that the standard 12-mile territorial limit is still in affect.

According to the transmissions from the yacht, the three Americans said "our vessel is being molested by armed vessels." Later they relayed that "our vessel is being rammed."

The passengers aboard the yacht Brilling identified themselves as Cornelia Anne Dellenbaugh, 28, of Vero Beach Fla., Charles Affel, 30, of Philadelphia; and Leland Dickerman, 29, of Flagstaff, Ariz.

The vessel is owned by Dellenbaugh, a 1971 graduate of the University of Pennyslvania who spent three years as a Peace Corps volunteer in Thailand and Malaysia before having the 35-foot Brilling built in Bangkok for $18,000.

According to Dellenbaugh's father, Frederick Dellenbaugh, an electronics engineer with Harris Corp., the three were on a leisurely trip back to the United States. "We expected them here by next Christmas," he said in a telephone interview from Vero Beach.

Herman Affel, prsident of Computer Consoles Inc., of Rochester, N.Y., said his son Charles became a sailing enthusiast after graduating from the University of Viriginia in 1971.

Dr. Joseph Dickerman, of North Hollywood, Calif., said his son had spent two years in the Peace Corps before joining Affel and Dellenbaugh two years ago for extended sailing throughout South China Sea.

The three have been engaged in by their parents as resourceful navisightseeing, boat deliveries and collection of specimen fish.

All three Americans were described by their parents as resoureful navigators. Dellenbaugh, who majored in social psychology and applied anthropology, took Naval ROTC program at the University of Pennsylvania. She and Dickerman both speak Thai and Malaysian.