In Heir to the Empire Thrawn makes it clear that he found Vader to be a weak, shortsighted, cruel, unthinking, and narcissistic. Just an attack dog that was too preoccupied with himself to think anything through. Thrawn also wasn't too happy with the emperor. Apparently Palpatine es very racist and only raised humans to the top ranks, which Thrawn found shortsighted and illogical. Thrawn was an exception to the emperor's bias because of how skill and effectiveness.

It's been a long time so it would take me too long to find the passages and paste it here.

EDIT: Heir to the Empire was the introduction of Thrawn so even though it got retconned into "Legends", I have decided to keep his attitude and mindset regarding Vader, Palpatine and the Sith as head cannon.