Secretary of State Cyrus Vance said today that many of America's NATO allies believe the consequences for East-West relations will be "disastrous" if the Senate fails to approve the U.S. strategic arms limitation treaty with the Soviet Union.

At a press conference concluding the spring meeting of NATO foreign ministers. Vance said that, during the private discussions, several West European ministers had said "they thought it would be disastrous if the SALT treaty is not ratified."

His comments about the impending Senate fight over approval of the SALT II accord came at a meeting that has been dominated largely by discussion about which direction the 15-nation alliance should be taking following the pact.

Although Vance did not identify the countries to which he was referring, it is known that all of the NATO partners except France generally support the treaty and that the major European members, Britain and West Germany, have given it especially strong backing.

In general, the West European governments have been persuaded to support the treaty by a belief that it does not impair the effectiveness of the U.S. nuclear shield over Europe and by popular sentiment within their respective countries for pursuing all possible detente initiatives with the Soviet Union.

The Carter administration, for its part, worked very hard during the long SALT negotiations to overcome whatever initial doubts were held by the NATO allies. Now, having successfully achieved that goal, the administration hopes to use it as a major arguing point to convince the Senate that failure to approve the treaty would damage NATO's morale and shatter U.S. credibility in East-West affairs.

Even the criticism expressed by France, which is concerned about the effects of SALT II on its independent nuclear force, was described by sources here as generally mild and proforma.

The final communique stopped short of putting NATO on record as endorsing the treaty because the full text, which is still being worked out, has not been studied by the various member governments. But the communique did say that the "ministers welcomed the agreement" and "agreed that . . . it will make an important contribution to East-West relations and security."

Among the questions raised by the treaty is whether the SALT III talks, scheduled to begin with the Soviets after the treaty is approved, will be broadened to include at least partial participation by other NATO countries.

That is because the United States wants SALT III to include bargaining on intermediate-range missiles based on the European mainland by NATO and the Soviet-led Warsaw Pact. Such negotiations would bear more directly on the internal politics of those NATO countries where these missiles are deployed than did SALT II, which was concerned with placing limits on intercontinental missiles.

Vance, while conceding that the possibility of broadened participation in SALT III is "certainly not ruled out," also said, "I think it's unlikely."

He added that the United States was discussing the question with its NATO partners but that "no consensus has been reached yet" on whether the West Europeans would like to take part or prefer to let the United States be the lone negotiator with the Soviets.

U.S. sources said privately that the Europeans seem to be leaning in the direction of staying out of SALT III, provided that Washington consults with them closely, and that the Soviets also would prefer to negotiate with the United States alone. For that reason, the sources added, the SALT III talks seem likely to remain a forum reserved to the two superpowers.

Because of the SALT agreement, the meeting here also spent a lot of time discussing ways NATO can pursue other arms control initiatives with the Soviet bloc. In particular, these included the current NATO-Warsaw Pact talks on reducing conventional forces in Central Europe and a recent Warsaw Past call for a European disarnament conference.

However, sources here have stressed that exploration of these ideas is still in a highly tentative stage - a fact reflected in the communique. In regard to calls for a disarmament conference, it said only that the ministers "decided to continue examining" such proposals.

On the question of conventional force cuts - known as mutual and balanced force reductions (MBFR) - the communique noted that NATO has made proposals to resolve the differces impeding the talks. It called on "the Eastern side to take full account of these Western moves and to respond positively in order to restore the momentum in the talks."

On another matter, Vance and other NATO officials said they expected the alliance to make decisions around the end of the year on proposals for modernizing NATO's theater nuclear forces - those using intermediate range missiles.

Washington has been urging such an improvement program on the grounds that successful negotiations about intermediate-range missiles in SALT III must be based on a position of strength. But, in some of the countries that would tentatively be asked to allow stationing of improved, longer range missiles such as a new version of the American Pershing, the matter has potentially serious political implications.

Vance is scheduled to pay a brief official visit to Spain Friday.