A model apartment is opening this week at the Baccarat Hotel and Residences, a 50-story tower under development on West 53rd Street and Fifth Avenue, across from the Museum of Modern Art. Singapore-based Pontiac Land Group is joining with Goldman Sachs Group Inc. and Hines to construct a 72-story condo tower on the same block. Farther east, China Vanke Co., China’s biggest publicly traded developer, is working with Aby Rosen’s RFR Holding LLC on a 61-story building on Lexington Avenue.