Whilst the Guardian and the Traitor says Vahlok had a one on one battle with Miraak which lasted days and just as Vahlok was about to win Miraak was snatched away, we are told in-game by the Shamans, Frea and her father, that the dragons swarmed Miraak's temple and destroyed him, they don't mention Vahlok, at all, even though they are meant to be his biggest advocates.
I think maybe it was planned for Vahlok to have a big role in the MQ, but this was later changed and parts of his story were just kept in as a minor quest, but now his story makes no sense and instead of Vahlok the Jailer we have Vahlok the Liar.