Welcome Fettclone1.
Personally, I reckon that Vader looks down on Fett and his cohorts with great disdain due to their way of life, and merely involves them as an additional way to locate Luke and the others a bit quicker. He already singles out Fett with his 'no disintegrations' order (probably due to knowing of his 'reputation')... but wouldn't deem it necessary to address these 'low life scum' on 'name' terms.