Yesterday's 2-hour transfer of Presidential power from George Bush to Dick Cheney while the former underwent a colonoscopy is a useful reminder that the office of Vice President was substantially altered by the adoption of the 25th Amendment in 1967. Under that Amendment, a President can temporarily lodge his authority in the VP whenever "he is unable to discharge the powers and duties of his office." (Insert Bush joke here.)
Under the pre-25th Amendment Constitution, it was plausible to argue that the VP was part of the legislative branch, since his only function was/is to break ties in the Senate. Sure, he would become President on the President's death, but the line of succession also goes to others (such as the Speaker of the House, next in line) who are clearly not members of the Executive Branch. But there is something qualitatively different between holding a legislative job that might result in a promotion to the Presidency and holding a job that includes, as one of its constitutional duties, occasionally acting as the temporary President. Add to this analysis the tacit recognition in the 12th Amendment that the VP will have been elected on the same ticket as the President, and it's no longer plausible to say that the VP is not part of the Executive. Amusing, sure, but not plausible.