Following Vane's success in negotiating the Scottish agreement, the death of John Pym at the end of 1643 propelled Vane into the leadership of Parliament, along with Oliver St John, Henry Marten, and Arthur Heselrige. He promoted, and became a chief member of, the Committee of Both Kingdoms, established in February 1644 as a point were English and Scottish authorities could coordinate war activities. Vane was then sent to York in June 1644, then besieged by three Parliament armies, to urge Sir Thomas Fairfax and the Earl of Manchester to divert some of those forces to face Prince Rupert of the Rhine, who had recently taken Liverpool and was pillaging properties of Parliamentary supporters in Lancashire. While there he also proposed to the generals the establishment of a government without the king. This idea was roundly rejected by the old guard generals who believed Charles could still be accommodated, but found support with the rising star of Oliver Cromwell.