He can't though, because it's already stated in the opening crawl that Vader is ruthelessly pursing the Rebel faction that Luke's with. He even states to Ozzel in the intro to him that, "And I'm sure SKYWALKER is with them."
Marcan and Gibbons where perfect in their set up. And Lucas in his own little way played into it more. So really this little tale improves with Lucas little adjustment in dialogue because it continues the little games Palpalatine and Vader are playing with each other.
Vader knows he has at the beginning of Empire, but the fact that Palpatine now reveals to Vader at this moment he too knows it well.. it's that lil exchange now heightened all thanks to this story. :)