President Carter, rebuffing persistent signals of friendship from Vietnam, is quietly extending the U.S. trade embargo against Hanoi.

In ist early months, the Carter administration showed considerable interest in friendly ties with Hanoi, but it now seems cool to the idea.

The major reason for the continuation of the embargo is that the administration is anxious not to offend China, which looks upon Vietnam as an instrument of Soviet strategic purposes in Southeast Asia, officials say.

And, they say, even though there have been indications from Vietnam that it no longer insists on a U.S. aid commitment prior to the establishment of normal diplomatic ties, there has been no official notice from Vietnam to that effect.

Under a law passed two years ago, the president is required each year to review restrictions on trade which have been imposed against various countries. He can lift such restrictions when he chooses.

A notice in Tuesday's Federal Register disclosed that the restrictions against Vietnam are being extended. Carter was rejuired to make an decision on lifting or extending the embargo by today.

Vietnam, plagued by conflicts with neighboring China and Cambodia, has been seeking outside diplomatic and economic support, and Washington has been one of its principal targets.

The administration says it is willing to exchange ambassadors with Vietnam without preconditions and that the trade embargo could be lifted immediately thereafter.

Officials concede there are sound economic arguments for lifting the embargo. Trade with Vietnam could help cut back on the U.S. trade deficit, expected to pass the $30 billionmark this year.

The Vietnamese are interested in U.S. oil technology, and their aviation and transportation industries, among others, are in need of replacement parts of American-made machinery left behind after the war.

The trade embargo against Vietnam is total except for certain hunanitarian items, such as food shipped by relief groups while the Treasury Department sometimes approves.