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.

I was interested too so I tried to search it up The only relevant entry I found was on the Star wars rebels wiki, which doesn't seem to support the claim but with how often this stuff gets retconed this could easily be out of date.

Thrawn and Vader first met during the Clone Wars on a Mission to Batuu, where they pursued the Separatists for their own reasons. Thrawn fought alongside Jedi General Anakin Skywalker, which left Skywalker impressed with Thrawn and Thrawn impressed with Skywalker. They later met again, but this time as Darth Vader. Due to Thrawn's meeting with Skywalker in the Clone Wars, Thrawn was able to figure out that Skywalker and Vader are the same person after witnessing similarities that they share. Thrawn was one of if not the only Imperial who wasn't afraid of Vader and was one of the few officers who actually respected Vader for his commitment and loyalty to the Empire. Vader in turn held great respect for the Grand Admiral's intelligence and strategic influence. Although, Vader's respect was somewhat tarnish for the Grand Admiral after his failure to capture the rebels on Atollon, Vader still respected and trusted the Grand Admiral during their second mission to Batuu to investigate a disturbance in the Force. Although Vader was fed up with Thrawn consistent trolling efforts to deduce that Darth Vader was in fact the Jedi General Anakin Skywalker he fought alongside so many years ago. When Thrawn started making the TIE Defender project, Vader was one of the officers who supported Thrawn's project and told Thrawn to make some modifications on it, which lead to the TIE Defender Elite.