''A small plus,'' commented one American diplomat, giving a mid-tour assessment of American Vice-President George Bush's 12-day European visit. In this analysis Mr. Bush, while not making any spectacular impact on European public opinion on missiles, has at least wrested the peace initiative back from the Soviets by his sheer presence and apparent reasonableness.
And this, combined with an upswing in conservative fortunes in the one country that matters most - pre-election West Germany - has gotten the US off the nuclear hot seat for the time being.
One measure of Bush's public relations success is initial confusion in Moscow. The Tass news agency issued two contradictory readings of Bush's trump of President Reagan's offer to meet with Soviet General Secretary Yuri Andropov to sign an agreement banning land-based Euromissiles. First, Tass thought that Reagan had undermined the popularity of West German Chancellor Helmut Kohl's government; later, however, Tass corrected itself to accuse Reagan of meddling in the West German election to help Kohl.
On his trip Bush is launching no new arms control proposal (apart from the announcement in Geneva that the United States would shortly submit a new package on a chemical weapons ban). The Reagan summit offer itself is conditional on Soviet acceptance of the Western ''zero option'' position at the American-Soviet arms control talks in Geneva - a position Moscow has already rejected because it would trade off future NATO missiles for the Soviet Union's presently deployed 333 SS-20s.
Despite repeating old positions, however, Bush has taken what he calls the ''high ground'' in promoting the zero option's would-be elimination of ''an entire class of weapons.'' He has further stressed that the US is not ''intransigent'' about the zero option. Any ''serious'' Soviet offer will be considered, Bush has said - and in front of the UN Committee on Disarmament Friday, he directly challenged the Soviets to come up with a compromise.
All this may not yet represent US flexibility that Western European leaders widely believe will come if the intermediate-range arms control talks are to get anywhere. But the Bush trip hints that American (and NATO) insistence on the zero option is a question of negotiating tactics and timing rather than a ploy to guarantee stalemate in the arms talks.