At is at least a footnote to the late Alexander Haig's long and distinguished career that his missteps played an important role in the rise of the modern vice presidency. Haig gave a speech in which he announced that he was the "vicar" for foreign policy. The Reagan administration, watching the growth of the National Security Council and its head in the Nixon and Carter administrations, did seek to downgrade the National Security Advisor and elevate the Secretary of State. But Haig really could not play nice with the other kids and seemed to think he was President of Foreign Policy. He tried to tell Reagan what to do and he fought with the National Security Advisor over running the crisis management group at the NSC.
The White House response was an interesting one. They gave the job to Vice President Bush. Giving into Haig would have looked bad and completely undercut NSA Richie Allen. But if Haig dug in his heels, Allen would not have been able to hold his own in bureaucratic combat with Haig. In giving the position to Bush the job went to someone that Haig didn't outrank.
Bush, by most accounts, was quite capable in the position. His best performance was on the day Reagan was shot. Haig severely tarnished an impressive reputation with his performance that day, where he gave a press conference announcing that he was in charge.
First off, he was actually wrong - succession does not go from the President to the VP to the Secretary of State (it did initially, but the Constitution was amended on this point.) The Speaker of the House and President Pro Tem of the Senate are in line first. Second, with the President and VP still alive succession does not occur at all. One can understand Haig's thinking. If the attack on the President were just part of a multi-pronged plan and enemies were to attack, someone would need to take command. But our ship of state is not ship, where someone has to be officially standing on the bridge.
Informally, someone does need to be coordinating affairs, but taking that stance publicly is not acceptable. It is surprising that Haig did not have a better sense of this, he had served as White House Chief of Staff in the waning days of the Nixon administration and many referred to him as the defacto president. He played this role without leaping into the limelight. (This had happened before. When Woodrow Wilson was laid up with a stroke the Secretary of Treasury ran cabinet meetings. When Wilson recovered, he promptly fired his rebellious cabinet officer.)
Vice President Bush, in contrast, was extremely prudent that day. He did not make excessive public statements and symbolically denied taking charge. When plans were made to helicopter him to the White House, Bush refused saying that only the President lands on the South Lawn. Bush took his motorcade to the Vice President's residence sending a signal to the American people that the government was functioning normally and not in crisis.
Thanks to Haig, both in his battles to control foreign policy and his overstepping when Reagan was shot, Bush received expanded opportunities to participate in the administration. While Reagan and Bush met regularly and Bush had a clear line into the White House through the Chief of Staff, a long-time friend, Jim Baker, it was not clear that this would have allowed him to play a substantial role. He was not seen as an ideological Reaganite - coming from the other end of the party. While Mondale established the modern vice presidency, it might have died with the Carter administration's failure to win re-election. Instead Bush was given the chance to manage an important national security role and later took on other national security tasks such as the Terrorism Commission which set policy and the South Florida Task Force.
The illustrious Christopher Hitchens illustrates some of my points about how Haig's power-grabbing led to George Bush's elevation to running the crisis management group. In other regards, Hitchens saw Haig as delusional and power mad (and he isn't alone in this assessment - the man worked with MacArthur and Nixon). He would probably think that I am being far too charitable to Haig and his infamous press conference. I still think Haig's military background, thinking that at all times someone has to be in the captain's chair, shaped the event.