Impeach the Bastards

So, yes, the President has an undoubted Constitutional power to grant pardons and commutations of sentence.

But suppose, just hypothetically, a pardon or commutation is offered as part of a conspiracy to secrete information relevant to an ongoing criminal investigation, or an investigation into actions that may provide the basis for impeachment of one or more high public officials (say, again hypothetically, a Vice President).

To get (very slightly) less hypothetical, suppose Mr. Libby's conviction rests on perjury and obstruction of justice aimed to cover up potentially impeachable actions of The Dick and/or his nominal boss, he of the initial "W". Suppose further that Mr. Libby's commitment to remaining silent was thought to be tested by his (rapidly) impending change of address. (Perhaps his wavering commitment was even communicated to persons allegedly serving in the "Executive Branch".) Finally, suppose that the commutation of Mr. Libby's "excessively severe" 30-month prison term (reduced to a somewhat less severe zero, perhaps with additional understanding that a full pardon would follow after the 2008 elections) would be understood to enhance the likelihood that Mr. Libby would not be inclined to discuss these matters further.

My understanding, as a non-specialist, is that the undoubted Constitutional authority of legislators to cast votes on matters before Congress does not in and of itself (pesky matters of the "speech or debate clause" aside, to avoid complicating matters in the purely legislative--and, of course, Vice-Presidential-- contexts) immunize such persons from investigation and conviction of bribery charges.

There remain, of course, some matters of evidence to be pursued. But isn't that what the House Judiciary Committee's investigative authority (pursuant to an impeachment inquiry) are for?

Or did I miss something in Civics class?