The Senate Foreign Relations Committee voted 10-9 Wednesday for a bill calling for a virtual trade embargo against South Africa, but the harsh measure has little chance of becoming law. All Democrats on the committee voted for the House-passed bill, and each Republican opposed it on grounds the measure will leave the next president no diplomatic leverage to force an end to the apartheid system of racial segregation in South Africa. Sen. Jesse Helms, R-N.C., pledged to lead a filibuster against the measure, making it unlikely the Senate would approve the measure in the rush to end the session in October before the general election.