As soon as Vader could hear Luke was with the Droid, he'd ask slipped off, to set up one of the refreshers. To lay out soap, towels, a mat on the floor. To gather up a robe (one of Skywalker's old ones, shorter in length and sleeve than Padmé wore, but of a similar construction and blue velvet) that he knew would be too big on Luke, but it would work.

"Follow me. Leave your clothing out, so it can be measured and cleaned," he comments as he hands over the robe.