I doubt any of them actually want to face Pence, who would possibly be a harder candidate to beat than a wounded Trump.
That has no bearing on their public position on the matter. The Dems have no power to remove Trump from office on their own, since there's zero chance that they're going to get a 2/3rds supermajority in the Senate this decade. It only happens if a large number of Republicans want to get rid of Trump. So Warren or Booker or Gillibrand voting to convict Trump and remove him from office (should such a vote ever come up) is a free vote, since none of them are going to be the deciding vote in his removal from office. So they'll vote based on what they think will get them the most votes for president, not based on whether they'd rather run against Trump or Pence in the general election.