WASHINGTON (AP) _ The American Civil Liberties Union says it will defend a pair of Ohio veterans if the government prosecutes them for arranging trips to Vietnam to heal ''old, deep wounds'' from the war they fought there two decades ago.

The announcement Thursday came just one day after the Treasury Department reaffirmed its intention to pursue a vigorous enforcement program against violations of embargoes that it says prohibit Americans from promoting or arranging unauthorized travel to Vietnam.

Susan R. Brenda, legislative analyst with the Washington office of the ACLU, said ''The Treasury Department is simply without authority to reach directly into the First Amendment-protected activities of Americans in the United States and prevent them from organizing towards the perfectly legal end of travel to Vietnam.''

Ms. Brenda appeared at a news conference with Donald Mills and John Myers, members of the Vietnam Veterans of America chapter in Akron, Ohio. The two men have organized six bargain-rate tours of Vietnam for veterans since 1984, and they said they will go ahead with another in November.

''We believe that an attempt to prosecute Vietnam veterans for traveling back to Vietnam in an effort to learn about present-day Vietnam and heal old, deep wounds would be a grave mistake,'' Ms. Brenda said. ''Should the government proceed along that course, however, we will assist these veterans in their legal defense.''

It is legal for Americans to travel to Vietnam, but the Treasury Department issued regulations last year making it illegal to ''arrange, promote, or facilitate group or individual tours or travel'' to any country listed under the Trading with the Enemy Act.

This list now includes Vietnam, Cambodia, North Korea and Cuba.

Mills and Myers began organizing tours of Vietnam after Mills revisited the country in 1984 as a member of the national board of Vietnam Veterans of America, which was looking into controversies over the defoliant Agent Orange and the issue of American servicemen missing in action.

Mills said that when he spoke to chapters of the organization upon his return, he found them mainly interested in hearing what Vietnam is like now.

Two veterans who have gone on the tours spoke glowingly of their experiences at the news conference.

''I found out that despite all of our efforts we weren't able to kill the whole country,'' said Greg Kleven, 40, of Concord, Calif., a former Marine. ''It's still alive. Alive and well.''

Mark McCausland, 38, of Munroe Falls, Ohio, an Army veteran who lost an arm and a leg in a rocket attack in Vietnam, said ''The image that I have of Vietnam now is of a peaceful green little country, where the image I had before was of a place that was deadly.''

''It would be a real shame to allow anybody to prevent other veterans from going through the same process,'' McCausland said.

McCausland said he believed it would be nearly impossible for most veterans to return to Vietnam on their own because of the expense and paperwork involved.

Myers said the organized tours last about three weeks ''from home town to home town'' and cost $2,700, including transportation, meals and accommodations.

Steven I. Pinter, chief of licensing for the Treasury Department's Office of Foreign Assets Control, wrote Mills and Myers on Feb. 10 directing them to ''immediately terminate performing any activities involving travel tours to Vietnam.''

The letter said violation of the regulations carries a maximum penalty of a $50,000 fine and 10 years imprisonment.

On May 15, Lindblad Travel Inc. of Westport, Conn., pleaded guilty in U.S. District Court in Connecticut to unlawfully arranging a group tour of Vietnam and Cambodia. Sentencing was set for June 14. One of the customers booking the tour was an undercover agent of the Customs Service.

In a news release about the Lindblad case on Wednesday, Salvatore R. Martoche, assistant secretary of the Treasury for enforcement, said, ''This case should send a strong message that violations of the Vietnam and Cambodia sanctions program will not be tolerated.''

Martoche said the Customs Service and Office of Foreign Assets Control ''will continue to work closely with the Department of Justice to pursue a vigorous enforcement program against violations of these and other embargoes.''