Boston University's faculty union voted yesterday evening to reject the administration's proposed contract renewal and to consider going on strike next term unless B.U. president John R. Silber presents an acceptable alternative.

Union members, who make up about half of the faculty, agreed in an open meeting to begin a campaign to pressure the administration to present an acceptable contract by soliciting the support of students, alumni, faculty, parents, and the media. Negotiations have been in progress since August, when the previous three-year contract expires.

Before 200 union members, Fritz K. Ringer, chief negotiator for the B.U. chapter of the American Association of University Professors, said there remain four unsettled items in the proposed package: salary increases, fringe benefits, merit-granting procedures, and chapter membership rights.

Before the vote, John N. Westling, an assistant to Silber, said the union's rejection of the contract would be a "mistake," adding, "It would be an error in their judgement, but that't their right."

Leonard Bloksberg, a negotiator for the union and a professor a Social Work said that he has "no way of knowing" whether the union will succeed in negotiating an acceptable contract before next term.

A faculty strike would affect 21,000 B.U. students--all except those in the Medical, Law, and Dentistry Schools. Most students interviewed said they supported the faculty, although they would attend classes in the event of a strike.

The six-year-old union gained official recognition in 1978 when members conducted a week-long strike to obtain their first contract.