U.S. PLANS TO KEEP ARMS PACT IN FORCE
Carter Adm tells Cong that it will continue to observe interim agreement limiting strategic arms so long as USSR also adheres to accord after it expires Oct 3; Vance, in lr to Sen Foreign Relations Com, says USSR has been told that US will issue statement of its own 'not to take any action inconsistent' with '72 accord; says he does not think statement will require Cong approval; lr is released as Gromyko ends his 1st day of talks with Vance on problems blocking new agreement; differences holding up accord cited; illus of Vance with Gromyko (M)