Vice President Bush is preparing to take on an assignment as diplomatic troubleshooter for President Reagan's controversial Strategic Defense Initiative, a research effort to build a high-technology shield against incoming Soviet nuclear missiles.

Bush has started briefings for a trip to Western Europe in which he will seek to ease allies' concerns about Reagan's missile defense program -- often called "Star Wars" -- and try to offset intensifying Soviet criticism of it.

The decision to send Bush reflects administration fears that the United States may have confused Western European leaders about the scope, cost and consequences of the $26 billion program.

"I think a lot of it is just being sure we're all reading from the same music," Bush said in an interview.

The trip also underscores the continuing Bush strategy of using the vice presidency as a platform from which to lay the groundwork for an expected 1988 presidential campaign. This would be his second major diplomatic effort of the year. Earlier, Bush visited famine-striken Africa and went to Moscow following the death of Soviet leader Konstantin Chernenko.

Although the timing of the European trip is uncertain, an advance team was there last week planning a visit that would include stops in Britain, France, West Germany, Belgium and the Netherlands.

Reagan's missile defense program was a major topic of discussion during last month's economic summit in Bonn, and there have been increasing signs of anxiety among Western European allies about their role in the program and its implications for their defense.

Unlike his high-profile European tour of 1982 to shore up support for the deployment of medium-range nuclear missiles, Bush said this time he expects to hold private meetings with Western European leaders. Among other things, he said he hopes to reassure them that "there is no intention" to "decouple" the U.S. nuclear deterrent from Europe if a missile defense system is perfected.

"Nobody's sitting off in a lab someplace saying, let's perfect this so we can live in a Fortress America," Bush said. "The principle is more noble than that. It is more moral than that. It leads inexorably down the line to the concept of why is it worth having nuclear weapons if you can perfect something to safely guard against them?"

Bush said he also hopes to allay allies' concerns about the technology and feasibility of the missile defense system.

On other topics:

Bush said Soviet leader Mikhail Gorbachev continues to express a desire for progress in the Geneva arms talks, which have just resumed, but has shown "no firm evidence, no hard evidence of that."

The vice president said tax reform could provide "a good shove" to Republicans in the direction of becoming the nation's majority party, but it's "too early to say that yet."

*

* A potential Bush rival for the presidency, Rep. Jack Kemp (R-N.Y.), has been pushing for a lower individual top rate than the 35 percent proposed by Reagan, but Bush said he thinks Reagan proposed a "balanced package" that "preserves incentives" for some businesses as well as reducing individual rates. Bush was instrumental in restoring tax breaks for the oil and gas industry in the final Reagan plan.

* Bush lamented that the 1988 campaign "began last August in Dallas because there wasn't much suspense down there," and said that "with '88 looming over the horizon a little more closely, people try to hyper-analyze it, interpret things differently."

* Bush said he is just "doing my job, the same as the last four years. . . . . I think that still remains the best approach, whether it is for doing the job or political. Sometimes no politics is the best politics." However, Bush has stepped up his political activity, having put together a new staff since his troubled reelection campaign last fall. A separate Bush political action committee, the Fund for America's Future, is in full swing, and Bush met privately with groups of Republican supporters in Phoenix and Baton Rouge, La., recently.