Vanguard Health Systems, the for-profit owner of St. Vincent Hospital in Worcester and MetroWest Medical Center in Framingham and Natick, said today it will partner with Tufts Medical Center to pursue the purchase of independent hospitals throughout Massachusetts, as well as pursue clinical relationships with other health systems. The new health system, as yet unnamed, will build upon an existing set of relationships between Tufts and MetroWest that already brings Tufts specialists in cardiology and pediatrics to the MetroWest campuses. The New England Quality Care Alliance, a 1,600-member doctor's group that is affiliated with Tufts Medical Center, will also be a part of the new system.