Also, Pence is a super-hedge toward impeachment. Believe it or not, Trump's picks of the SCOTUS would pale in conservative-ness compared to anything Pence would do. For this reason alone, I don't think anyone on the left (in congress) wants him impeached. Add to that what Isildur said about the majority it will take and it just isn't likely.
That said, I have heard a compelling argument or two that Estonian/Ukranian/Latvian intelligence has some pretty good stuff on him that could come out if they wanted it to. I don't know about the specifics, but those nations supposedly have some very good intelligence agencies when it comes to Russia and they've been insinuated to have some dirt-- especially if Trump keeps digging himself in with "nothing ever at all to do with Russia".