Eight Vietnam veterans who came to Washington last month promising to go without food until the government acted on several demands, officially ended their fast yesterday in Lafayette Square near the White House, claiming to be satisfied with congressional responses but disappointed with the Reagan administration's.

The hunger strikers came to Washington June 16 seeking federally financed research into the long-term effects on Vietnam veterans of toxic herbicides such as Agent Organge. They also sought approval of a civilian readjustment program for veterans suffering from "delayed stress syndrome" and an independent investigation into the death of James Hopkins, a Los Angeles veteran whose death from a drug overdose sparked the hunger strike. In addition, the strikers asked for a meeting with President Reagan.

According to John Avalos, president of the California-based Veterans' Coalition, the group is satisfied with an agreement by the House Veterans Affairs Committee to consider the need for additional funds to study toxic herbicides used in Vietnam and to hold hearings on the Veterans Administration's treatment programs for Vietnam veterans.

"We're giving them one year from now to prepare a new budget and implement changes," said John Rigole, coalition spokesman.

To mark the end of the fast yesterday, the strikers and about 50 supporters gathered at Lafayette Square at noon for a communion service during which several of them gave thanks for public and congressional support and complained about the Reagan administration's "indifference to the nation's veterans."