Others have asked what legal justification there can be for impeaching Trump only when Pence to all appearances seems to have been involved in the Ukraine scheme (along with Secretary of State Mike Pompeo and Attorney General William Barr).
The answer to that is simple. In the end, impeachment is political, not legal. Politically speaking, it just is not possible to impeach the President, the Vice President, the Secretary of State, and the Attorney General. The idea is simply too absurd to consider, especially since that would make Nancy Pelosi President.
Donald Trump is clearly the evil genius behind the whole Ukraine scheme.
Donald Trump has also given us more than grounds to remove him under the 25th Amendment (for removing a President unfit for office). Mike Pence has not. Donald Trump is becoming increasingly unglued under stress and may burn the whole thing down. Mike Pence inspires no such fears.
And finally, I will quote Matthew Yglesias on why everyone across the spectrum should vote for Hillary Clinton, with just a few nouns and pronouns changed:
[Mike Pence], for all [his] flaws, has demonstrated a basic level of competence. [He] understands how policy and government work. [He's] not openly racist; [he] hasn't encouraged street violence. There's no risk that [he] would disrupt the international order or cause an economic crisis out of pique.
That's a really, really low bar. But it's the only bar [he] has to clear in this contest.