If the new Soviet regime plans to be less adventuresome militarily than the old, "perhaps the first indication" will come from Afghanistan, says Gen. John W. Vessey Jr., the chairman of the Joint Chiefs of Staff.

Sharing his view of what the Soviet Union will be like without President Leonid I. Brezhnev, he cautioned Pentagon reporters at breakfast yesterday not to look for anything startling at the arms control table in Geneva at least until spring.

"It would be difficult for the Soviets to move off" their current negotiating position until then, Vessey said.

The Soviets have 100,000 troops in Afghanistan and are no closer to winning than they were three years ago when they invaded a country that has defied many would-be conquerors, Vessey said.

It's too bad, he continued, that the Soviets didn't study what happened to the British when they tried to take over Afghanistan.

"If they had," Vessey said, "they might not have gone in."

He was asked: could the Soviets win now that they are in Afghanistan?

"I don't know," he replied. A better idea, he added, would be "to give the order to march out."

Vessey expressed surprise at "the quickness of the announcement" by the Kremlin that Yuri V. Andropov, 68, will succeed Brezhnev, who died Wednesday of a heart attack at age 75, as general secretary of the communist party.

Vessey, 60, goes back to the World War II days when Russia and the United States were allies. He fought in World War II, first as a mud soldier, then as an officer who won a battlefield commission during the fight for the Anzio beachhead in May, 1944. Now he holds the highest-ranking job in the American military.

A reporter asked if the United States could win a war with Russia.

"I don't think we should march out and get in one," Vessey responded. "I'm confident our forces would do very well. I would not go into a war with the Soviet Union comfortably. We have no plans to go into a war with the Soviet Union."

But, a reporter pressed, would the United States win if it did go to war with the Soviet Union?

"I don't know whether we'd win or not. What's your measure of winning? We don't want a war."

Although combating the Soviet threat will be Vessey's main concern as he makes his recommendations to President Reagan on the defense budget, Vessey said that what looks "most dangerous" as far as war breaking out is the Third World. "Our own backyard," said Vessey of Latin America, "causes me considerable concern."

Vessey also fielded questions for two hours on other situations confronting the American military:

* The Iranian-Iraqi war. "I don't see much evidence" that the Soviets are moving into Iran in worrisome numbers to exploit the situation. "There's no love lost between the Iranians and the Soviets."

* The Sino-Soviet thaw. "I don't see China and the Soviet Union becoming great friends."

* Expensive weapons. "I think we're generally doing the right things. We're stuck with the general size of the force" remaining about the same, meaning that military leaders must rely on quality to offset the Soviet advantages in quantity of troops and weapons. "Nobody wants to march out on the battlefield with inferior weapons, especially when you're outnumbered."

Vessey said the M1 main battle tanks, which cost $2.7 million each, and the F15 and F16 fighter planes, are proving in additional performance that they are worth their high costs.

As for the Navy's F18 attack plane, which Navy pilots fault for short range and other flaws, Vessey said he had not read any critical report.

"That's not my job. I have confidence that whatever is wrong with the F18 can be fixed. If they've got a turkey, it ought not to be produced . . . At the right time, I'll get into the F18." Pentagon leaders are scheduled to review the future of the $40 billion F18 program on Nov. 30.

The Army's Pershing battlefield missile destined for Europe, which the Pentagon has ordered into production despite failures in flight tests, is in a "crash program. We know enough to be able to fix whatever is wrong with it." A flight test scheduled for today was canceled yesterday because of problems in the missile's electrical circuits, the Pentagon said. The Pentagon announcement was made after Vessey made his remarks to reporters.

* Military retirement. Although reforms of this multibillion-dollar program have been recommended for years, no administration has changed it basically. The system allows a military person to retire at half pay after 20 years service and at three-quarters pay after 30 years or more. Vessey said he was open to suggestions for changes that would apply only to people coming into the service in the future.

* Resuming the draft. As long as the government pays officers and soldiers comparably to civilian employers, and provides other benefits, "I don't see the need" for imposing conscription in peacetime.

He added that he was pleased with the quality of the services' recruits and the fact that they are not representative economically and racially of the nation's youth population "doesn't bother me."

He said recent enlistment patterns are making the all-volunteer force "more representative."