“... who presides if a Chief Justice is the one being impeached?”
From that Constitution thingy: “The Vice President of the United States shall be President of the Senate, but shall have no Vote, unless they be equally divided. “
The more interesting question might be: “Who presides if the Vice President is the one being impeached?”
Here’s a link to a short article on the topic.
https://www.lawliberty.org/2012/10/26/7350/
Article I, § 3, cl. 6, gives to the Senate “the sole power to try all impeachments,” requires that Senators be under oath or affirmation when sitting for that purpose, stipulates that the Chief Justice of the United States is to preside when the President of the United States is tried, and provides for conviction on the vote of two-thirds of the members present
However, rereading it again I realized the Chief Justice only presides of impeachment for the President only, not all impeachments. I hadn't read it in awhile. 8>)
Concerning the impeachment of a Vice President, a VP can only vote to break a tie. Since conviction requires two-thirds of the Senate voting to conviction there would be no tie breaker. But who does preside over a VP impeachment, still remains unanswered, Constitutionally. 8>)