A partnership between New York real estate investment firm Vantage Properties and Philadelphia private equity firm Lubert-Adler went into contract last week to buy Rego Park’s largest apartment building for about $75 million, a source close to the deal said. The property, known as Saxon Hall, is in contract for $180,000 per unit. Another possible buyer, Taconic Investment Partners, had considered a plan to convert the 417-unit rental building into condos, but the idea was abandoned. The building will remain a rental property under Vantage Properties, the source said. The high-rise at 62-60 99th Street is three blocks from the 600,000-square-foot expansion of the Rego Park mall, which is being built by Vornado Realty Trust. Vantage Properties, which owns thousands of apartments in Manhattan and Queens, bought a portfolio of about 50 rent-stabilized buildings in Queens in January for $300 million.