Question is where did all those spare parts and ammunition go? They don't expire just because aid was cut back.
During the 75' invasion, ARVN was trying to reload spent brass and was even washing and re-using bandages. Ammunition expenditure was strictly rationed.
Not nearly enough gas either but this can be blamed on the mid 70s oil crisis.
June 19, 1973 - The U.S. Congress passes the Case-Church Amendment which forbids any further U.S. military involvement in Southeast Asia, effective August 15, 1973. The veto-proof vote is 278-124 in the House and 64-26 in the Senate. The Amendment paves the way for North Vietnam to wage yet another invasion of the South, this time without fear of U.S. bombing.
December 13, 1974 - North Vietnam violates the Paris peace treaty and tests President Ford's resolve by attacking Phuoc Long Province in South Vietnam. President Ford responds with diplomatic protests but no military force in compliance with the Congressional ban on all U.S. military activity in Southeast Asia.
July 17, 1973 - Secretary of Defense James Schlesinger testifies before the Armed Forces Committee that 3500 bombing raids were launched into Cambodia to protect American troops by targeting NVA positions. The extent of Nixon's secret bombing campaign angers many in Congress and results in the first call for Nixon's impeachment.
April 21, 1975 - A bitter, tearful President Thieu resigns during a 90 minute rambling TV speech to the people of South Vietnam. Thieu reads from the letter sent by Nixon in 1972 pledging "severe retaliatory action" if South Vietnam was threatened. Thieu condemns the Paris Peace Accords, Henry Kissinger and the U.S. "The United States has not respected its promises. It is inhumane. It is untrustworthy. It is irresponsible." He is then ushered into exile in Taiwan, aided by the CIA.