Am I missing something?
Yeah, Vader is lying to the Emperor. The film never explicitly states when Vader figures out who Luke is, but it’s obvious he’s well aware before the Emperor tells him.
I mean, it makes sense in light of the whole “join me and we can overthrow the emperor” speech at the end… Vader is plotting against the Emperor throughout the film.
JEDIT:
And he doesn’t really care about the Death Star. He expressed his disdain for it in ANH and in ESB he is on a dogged, single-minded mission to find Skywalker. Decimating the rebels is just a byproduct. It’s quite clear that Vader’s one and only goal throughout ESB is finding and capturing (and turning) Luke. And he’ll do anything (wipe out the rebel army, fly the entire imperial fleet through an asteroid field, lie to his master, take over a mining colony, etc.) to accomplish that one goal.