Vertex Pharmaceuticals Inc. said it has entered into deals with GlaxoSmithKline and Janssen Pharmaceuticals to work on potential treatments for hepatitis C. Cambridge-based Vertex already has a hepatitis C drug on the market called Incivek. The separate studies with the British drug company and Janssen, unit of Johnson & Johnson, call for VX-135, another Vertex hepatitis C drug candidate, to be tested in combinations with pills made by those firms. Those studies are scheduled to start early next year.