Nine Republican members of Congress who are sympathetic to President Reagan's request for $100 million in aid to rebels trying to overthrow the Sandinista government here met today with Sandinista leaders in what they called a fact-finding mission.

''This is a delegation that has come to listen and learn in hopes that what we see and hear we will be able to plug into the debate about to take place in Congress,'' Rep. Robert S. Walker of Pennsylvania said. He added that ''most of us here are committed to voting for aid'' for the rebels, also known as contras, or counterrevoluitionaries.

Both houses of Congress are scheduled to vote on the administration's request next week.

After meeting with the congressmen, Nicaraguan Vice President Sergio Ramirez expressed some skepticism about the importance of the visit, saying:'' ''I don't think that the fact that nine congressmen came to Nicaragua to confirm the opinions tha they had before they arrived is going to have much effect on either the North American press or North American public opinion.''

Deputy Foreign Minister Victor Hugo Tinoco and Saul Arana, head of the ministry's North American desks, also attended the meeting.

One Sandinista official, who asked not to be identified, called the mission a ''propaganda trip.''

Rep. Robert K. Dornan of California said he suggested to Ramirez that three sets of Central American negotiations be opened simultaneously: between Salvadoran President Joe Napoleon Duarte and rebels opposing his government; between the Sandinista and the contras, and between the U.S. and Nicaraguan governments. He said Ramirez's response was noncommittal, adding: ''I don't think he wants to give me an answer until he sees the vote.''

''I wanted to make one more plea in the name of the president and our secretary of state to start negotiations,'' Dornan had told reports earlier.

The United States has called for negotiations between the Sandinistas and the rebels, but the Sandinistas consistently have refused such talks, calling instead for negotiations between the Nicaraguan and U.S. governments.

Rep. Paul B. Henry of Michigan said, ''I have many people in my district who have expressed concern with my position . . . I am here to give the benefit of the doubt to those that would question that action.''

The delegation also included several Defense Department officials, a White House aide and three officials from the State Department, including Deputy Assistant Secretary of State for Inter-American affairs William G. Walker.

This morning, the visitors stopped by the opposition newspaper La Prensa. They also were scheduled to meet with other major opposition figures, including the directors of an independent human rights group, with leaders of the opposition political parties and with Cardinal Miguel Obando y Bravo, the Sandinistas' strongest opponent within the Roman Catholic Church hierarchy.

The congressmen arrived here in an official U.S. plane. A U.S. diplomat said the plane had some difficulty getting permission to land at Augusto C. Sandino International Airport here. It flew on to San Jose, Costa Rica, to spend the night, and was schedule to take the congressmen to El Salvador Saturday tob continue their fact-finding mission.