Testifying for a second day in favor of the new strategic arms limitation treaty, Secretary of State Cyrus R. Vance said yesterday that the United States could survive if the Senate rejects the treaty, but the North Atlantic Treaty Organization might not.

Those two observations summarized the tone of Vance's testimony. On one hand he sought to convey an appreciation for the Senate's role in the treaty process; at the time he drew a grim picture of a world without SALT.

Rejection of the treaty would "cast a chilling shadow over the whole range of East-West relations," Vance said, increasing the dangers of the inevitable competition between the Soviet Union and the United States, jeopardizing all other arms control negotiations, undermining NATO and perhaps encouraging the spread of nuclear weapons to countries that don't yet have them.

Vance said that rejection of SALT would neither alleviate any American problem in the world nor enhance American security. Approval of the treaty, on the other hand, he said, would permit the United States to make what he called improvements in its strategic forces and improve the prospects for a more stable world environment.

For the second consecutive day, members of the Senate Foreign Relations Committee gave Vance, and later other administration witnesses, a relatively friendly hearing.

The most serious negative comments came from senators who feel the treaty would do little to control the arms race, particularly George McGovern (D-S.D) and Edmund S. Muskie (D.-Maine) The three Republicans who have identified themselves as critics of the treaty did not directly challenge its provisions yesterday.

Vance testified in the morning. In an afternoon session the committee heard Gen. George M. Seignious II, director of the Arms Control and Disarmament Agency, and Ralph Earle II, chief SALT negotiator.

Seignious gave the briefest and most forceful prepared testimony of any of the Carter administration officials who have appeared before the committee thus far.

"In the nuclear age," Seignious told the committee, "SALT provides a rational solution to the most difficult task that faces a strategist: to find some way to limit the forces of your opponent. With no agreed limitation no matter how many weapons of our own we build, we cannot stop a Soviet buildup. And with nuclear weapons, against which there is no adequate defense, the result of such a race is not more security for one, but greater insecurity for all."

Once again many senators spoke of their desire to add understandings, reservations and perhaps amendments to the treaty. Joseph R. Biden Jr. (D-Del.) even offered a list of eight reservations or understandings that he thought senators on the committee were inclined to support. The chairman, Frank Church (D-Idaho), indicated separately that he favored many on Biden's list.

Many on the list already have been endorsed by other committee members. The list suggests understandings that would:

State that acceptance of SALT II should not be seen as American acceptance of "Soviet aims and conduct" around the world;

Declare America's intention "to maintain essential equivalence with the Soviet Union".

Label SALT II an "interim measure" that should lead to further arms reductions;

Declare America's intention both to maintan "traditional patterns of military and technological cooperation" with its NATO allies and to build a mobile, land-based missile system despite SALT 11;

Interpret as legally binding Soviet assurances on production of the Blackfire bomber and all the "agreed statements and understandings" that accompany the treaty, and

Establish that the protocol accompanying the treaty, which limits cruise missles and mobile missiles, cannot be extended beyond its expiration in 1981 without Senate approval.

There are much more modest changes than critics of the treaty later will propose. A strong consensus seems to be emerging within the Foreign Relations Committee that changes at least along the lines of Biden's list ought to be made before the treaty is sent to the Senate floor.

The subject of U.S. arms aid to NATO under SALT II received renewed attention yesterday. Vance gave the committee the text of a statement the United States read to its NATO allies 10 days ago assuring them that the treaty would not interrupt the traditional American relationship with them.

Specifically, the United States told its allies that the noncircumvention provision in the SALT agreement, which prohibits either signatory from evading the pact through a third party, "will not affect existing patterns of collaboration and cooperation with its allies, nor will it preclude co-operation in modernization" of allied armed forces.

Sen. Jesse Helms (R-N.C.), an opponent at the treaty, asked Church to call a closed session to discuss this issue. Helms indicated he had information which the administration disputed , that the Europeans are not satisfied.

Critics of SALT II who feel it is too weak yesterday vigorously cross-examined both Vance and Seignious. McGovern told Vance he could not believe that the secretary could support the MX missile, which he called "an obvious boondoggle."

Muskie pressed Vance to give reasons anyone should be optimisitc that a future arms limitation treaty would produce reductions in the superpowers' arsenals when both SALT I and SALT II have permitted substantial expansions.

Vance replied that arms control must be an ongoing process. Muskie then said he though it was "appropriate to note" that the secretary had not given him any reasons for optimism about a potential SALT II. He said the United States may have been too eager to get this agreement and thus too willing to sign a pact that failded to reduce nuclear weapon arsenals.

McGovern asked Vance if it meant anything to be "superior" in nuclear arms of both countries retained the power to destroy each other. Vance said, "We have to deal with the issue of perceptions" -- what other nations may think about the relative power of the two superpowers.

"Can you cite a case where those perceptions made any difference?" McGovern asked.

Vance replied that the NATO allies "are concerned about perceptions."