Should Impeachment Be Off the Table?

Ethan Leib has an interesting piece on Nancy Pelosi's statement that impeachment of President Bush is "off the table" if she becomes Speaker of the House. Leib goes through the constitutional analysis and then suggests that Pelosi is letting down her San Francisco constituents. Here's another thought: Any impeachable offenses committed by Bush were likely committed by Cheney as well, so that impeaching Bush alone would not get the job done (assuming one thought this a job worth doing). But to impeach Bush and Cheney simultaneously would smack of partisanship, since it would leave Pelosi herself as President. (The Speaker is next in line after the VP). Accordingly, following the pattern that made Ford President, Cheney would have to go first, at which point Bush would name a new, untainted VP (McCain? John Warner?), and then Bush himself would be impeached.
Yes, I realize this is utterly idle speculation. Even if the Dems take the Senate on Tuesday, they'll never have the margin to pull this off, and it would be a terrible political move--which is why Pelosi has taken impeachment off the table. I'm just interested in the impeachment mechanism more broadly, and how it contains this flaw, given that in many circumstances in which Presidential impeachment will seem warranted, so will Vice Presidential impeachment. (Note that in the Framers' day, pre-12th Amendment, this would have made more sense, because the Prez and VP would have been less closely associated politically -- or, as with Adams and Jefferson, actual political opponents.)