Ashok Vaidya thought his bill for two cortisone injections in April 2011 at Meadowlands Hospital and Medical Center was settled long ago — until he received a collection notice recently for $86,846. The computer programmer from Middlesex County is one of dozens, perhaps hundreds, of patients being dunned for charges the Secaucus hospital says should have been paid by UnitedHealthcare. The for-profit owners of Meadowlands, who took over in December 2010, say they were not bound by the rates agreed upon between the previous owner and the insurer.