Aides Say U.S. Never Endorsed Israeli Proposal
Vance says his Feb 10 comments were mere repetition of views expressed publicly before; State Dept official says Carter Adm did not endorse Begin's peace proposal, but simply found it a possible basis for negotiations; Dayan, in int with NY Times, raises questions about ability of US to continue as neutral intermediary; still plans to meet with Vance on Feb 16 (M)