Residents of the West Village will soon see something unusual arriving at the shiplike building on Seventh Avenue that used to house part of St. Vincent’s Hospital: ambulances. Four years after St. Vincent’s closed, the hulking white building, between West 12th and West 13th Streets, is reopening in the coming days, not as a hospital, but as a free-standing emergency room. The new E.R., however, is part of a trend that has as much to do with a hospital’s bottom line as it does with providing acute care.