To answer my own unasked question:

From a memorandum to the Attorney General written by Randolph D. Moss, Assistant Attorney General, dated August 18, 2000:

In 1972, the United States Attorney for the District of Maryland empaneled a grand jury to investigate criminal charges against Vice President Spiro Agnew. The Vice President filed a motion with the district court supervising the grand jury seeking to enjoin the grand jury from investigating or indicting him, claiming that his office gave him immunity from indictment and criminal trial. The United States filed a brief, signed by Solicitor General Robert Bork, opposing the Vice President's motion. The brief's central contention was that "all civil officers of the United States other than the President are amenable to the federal criminal process either before or after the conclusion of impeachment proceedings." Memorandum for the United States Concerning the Vice President's Claim of Constitutional Immunity, In Re Proceedings of the Grand Jury Impaneled December 5, 1972: Application of Spiro T. Agnew, Vice President of the United States, Civ. No. 73-965 (D. Md. filed Oct. 5, 1973) at 3 ("Agnew Brief").

Maybe it's just corrupt Republican Vice Presidents, but that'll work. It seems Professor Bainbridge has the same memo.

(Via Comments at Daily Kos.)