Many Trumpers are claiming that this is the Dem’s evil plan. (“Coup.”)
Technically, it’s likely to be possible only if the Pres and VP die in a plane crash or something. Because, as what happened with Agnew and then Nixon, the impeachment process is separate; if Trump gets removed or resigns, the VP immediately becomes President and selects a new VP. If Pence is then impeached and removed, Pence’s VP becomes President.
With Nixon, Agnew resigned and then Nixon made Ford his new VP, then Nixon resigned, then Ford became President (and he was not elected at all).