It's always nice to be noticed, and Vice President Bush is doubtless delighted with the prospect of his European trip and the prominence given it by the president. There's only one flaw: having to explain Ronald Reagan's policies on nuclear disarmament makes it Mission Impossible.

No one knows what the president's true feelings are about negotiating an arms agreement with the Soviets. It almost depends on the day of the week.

The president has learned to profess that he wants genuine arms reduction. But last week he wrote a hair-raising letter to Rep. Jack Kemp (R-N.Y.) in which he said that if he doesn't get the MX missile he may call off the arms talks in Geneva.

Sen. Gary Hart (D-Colo.) rightly called it "blackmail."

One of the vice president's more unenviable tasks will be to explain to our European allies what the president really meant.

The vice president has, in recent months, all but disappeared from public view, which of course is what vice presidents are supposed to do.

Heirs-apparent provide unwelcome reminders of presidential mortality, and Bush, whose record of self-effacement is impeccable, has been made to feel that he must do even more in this regard because of the rage the sight of him stirs in the right wing.

It speaks something of the president's plight that he had to call on Bush to become a human umbrella between him and the peace initiatives that Soviet leader Yuri V. Andropov has been dropping like yellow rain on the White House since he took office in November. The West German elections come up on March 6, and Andropov seems to be trying to turn them into a referendum on Reagan's sincerity on the subject of arms control.

Offers of reductions in arms, invitations to the summit, suggestions of a non-agression agreement between NATO and Warsaw Pact nations, pour out of Moscow. The president ducks and dodges, alternately grudging and genial as he tries to fend off the Soviet dove.

The question Bush will be asked behind the scenes of his progress through Europe: is the president serious about halting the arms race?

The answer could only be yes and no. Obviously the president needs a victory.

His economic recovery program is in ribbons. He is urged publicly by his own people to cut the defense budget, which is at the heart of his "peace through strength" approach to disarmament.

What better guarantee of posterity than to make the world a safer place? What greater tribute to the cause of conservatives than to show they can succeed where liberals fail?

The trouble with an arms agreement from Reagan's point of view is that it must be signed with the Soviets. If only President Marcos, for instance, instead of Andropov would have pen in hand, it would be a piece of cake.

Bush is the only member of the administration besides Secretary of State George P. Shultz who has credentials in European capitals, where politicians quake at the thought of deploying new missiles in the absence of progress at the Geneva talks.

"He is thought of as a non-wild man," said a European diplomat cautiously of the vice president.

During the 1980 campaign, Bush rattled liberals with his claim that nuclear war is survivable.

Since then, he has said nothing provocative on the subject. He has not joined the president in characterizing advocates of the nuclear freeze as dupes or dopes. He has not berated bishops for failing to understand the Reagan formula for stopping the arms race by accelerating it.

He got a good notice from the New Yorker--maybe too good from the White House point of view--about his conduct at the Brezhnev funeral in Moscow. He was called "statesmanlike, serious, friendly, gracious . . . very much the handsome, good-natured American."

His remarks on that occasion about the possibility of starting over and the chance for the superpowers to drop "their fears, suspicions and distrust" gave some hope of a more rational approach--except that the president on the same day, in Washington, was warning the West German chancellor of the Red Menace.

Bush's statement, it turns out, had been cleared by his traveling companion, Shultz, who, mysteriously, steers clear of nuclear matters. The statement was not submitted to the White House hard-liner, national security affairs adviser William P. Clark, who doubtless crafted the president's bellicose text.

Maybe the most constructive thing the vice president could do on his dicey journey is to tell European leaders in private that they must sign up in the war for the president's mind on arms control. People here who try seem to succeed only every other day.