Even if both Trump and Pence were impeached
They wouldn't be impeached at the same time. If Trump was impeached first, Pence becomes president and gets to appoint a vice president (with senate confirmation, which would be easy since there is a republican majority)
Presumably he wouldn't pick someone up to his/her neck in Trump's Ukraine swamp like Pompeo, so even if Pence was impeached next that new VP would become president. Impeachment could never go down the succession line because even if republican senators found their balls and realized the depth's of Trump's crimes they'd never agree to back to back removal votes that left no room for Pence to appoint a VP.