SilverWook said:

It's been speculated Vader prevents Boba from shooting Chewie in ESB because Threepio is on his back, but that's a stretch. A Dark Horse comic does have Vader recognize him, and he gives the order to have the blasted droid put in the cell with Chewie.

I have wondered if Artoo can see Obi Wan's ghost...

Vader is ok with killing younglings, a planet full of people, his old besty Ben and his own son (the last remaining remnant of padme) but he's sympathetic to a droid that he left without hesitation as a kid?? I dunno...