To Impeach all they have to do is vote on the Articles. How they get there is entirely up to the Democrats. There is no reason to vote, it will not give their subpoena any more authority to compel documents and testimony than it already has. Mueller was not working for the House nor did he report to the House. The Republican controlled the House and the Democrats have had no investigatory power as the minority. They have been in control for less than a year.