The question can be argued either way, but the better view is that Pence, too, would be impeachable. The reason is that, had Trump not engaged in electoral fraud and corruption, Pence, like Trump, would not have been elected. That Pence would still be first in the line of succession to replace Trump is the result of an unintended consequence of the
12th Amendment, which was ratified in 1804. The fate of the Republic ought not turn on a constitutional oversight.
Before the amendment’s ratification, the original Constitution permitted removal of a president for electoral fraud. It didn’t matter whether that misconduct occurred before or after a president took office. The Framers clearly intended that theft of an election through fraud and corruption constitute an impeachable offense.