Chinese Vice Premier Teng Hsiao-ping told a group of American reporters today that if the Soviet Union should come into the Sino-Vietnamese war to help its ally, "we are prepared against them."

In some of Peking's toughest talk yet, the Chinese leader, in good spirits and a cocky mood, nevertheless said that he doubted "that the Soviet Union would take too big an action. If they should really come, there is nothing we can do about it... I think our action is limited and it will not give rise to a very big event."

Teng roundly denounced Vietnam as "a Cuba of the Orient" which has been "swashbuckling" around Asia, "emboldened by the so-called tremendous force which is backing them." This allusion to the Soviet Union was only barely disguised.

Teng talked freely to American reporters here to cover the negotiations under way with Treasury Secretary W. Michael Blumenthal for expanded trade with China. The improptu press conference took place in the south foyer of the Great Hall of the People as Teng waited to greet Blumenthal for the first meeting between the two men since Blumenthal arrived Saturday night.

The vice premier delivered his blast against Vietnam and its Soviet backers although well aware that Blumenthal was about to give him a welladvertised plea from President Carter to exercise moderation and bring hostilities to an end.

Blumenthal later told reporters that he had asked Teng, on behalf of the U.S. government, to withdraw "as quickly as possible from Vietnam," adding that Vietnam should also withdraw from Cambodia. The treasury secretary, the first high U.S. visitor here since the beginning of the Sino-Vietnam border conflict, told Teng that Washington opposes "the solution of international disputes by force or violation of borders."

The secretary said he had communicated Teng's response to President Carter and would not reveal it to reporters. But there was nothing to indicate that it differed from the message Teng had given the reporters moments before.

Asked when Chinese troops would withdraw from Vietnam, Teng said the real question is why Vietnam had sent troops into Cambodia and why Vietnam should have been allowed "in the past few years to commit incursions into Chinese territory several hundred times a year."

Teng said China would teach Vietnam a "limited lesson. It will not last a long time." But in the course of teaching Vietnam a lesson, he said, the myth of Vietnamese invincibility, and "the claim that it is the third strongest military power in the world, would be shattered."

He appeared to take a step back from assurance he gave yesterday to a Japanese interviewer that China would support a U.N. resolution calling for a simultaneous withdrawal of foreign troops from Vietnam and Cambodia.

Asked if the withdrawal of Chinese troops from Vietnam is "tied" to the withdrawal of Vietnamese troops from Cambodia, Teng said: "There is reason to connect the two things together but we are not prepared to do that. We do not want to do that."

Teng was asked whether he was concerned that world opinion might turn against China, as Blumenthal warned Sunday. His response:

"It seems to me that at present there is more favorable public world opinion. We Chinese hold that this action is highly necessary. We cannot tolerate a Cuba -- Cubans -- to go swashbuckling unchecked in Africa, the Middle East and other areas. Nor can we tolerate the Cuba of the Orient to go swashbuckling in Laos, Cambodia or even into the Chinese border areas. And I think the people of the world should not tolerate such action."

Teng ducked a question on the extent of casualties, admitting that there would be some in any war. Then, to the amusement of his assembled staff, he said that "as regards the estimate of the size of forces involved, your satellites offer accurate information, and that's about the size of it."

Blumenthal is to visit with Communist Party Chairman Hua Kuo-feng on Wednesday.