It's pretty obvious to Darth Vader that Luke as been taught by Obi-Wan in the arts of the force and lightsaber dueling. If you'll remember that from Vader's point of view (either pre PT or post PT) Obi had Luke and was training him. Vader was impressed that Obi-Wan could teach Luke all those great Jedi moves, arts, Jedi logic, and Jedi theology. Now it is times for Vader to make a move on Luke.

So when Vader says"Obi-wan has trained you well..." you'd better prepare for "...but I can teach you what he cannot the greatest power known to man lies on the path to dark side. Join me, my son, and together we can rule galaxy as father and son."

It's a sales pitch from Vader.

So when you take your car to a dealership

Salesman: "Toyota has made you a well built car......., but I can sell you a 2007 Chevrolet pickup which will of the ameneties for 75 cents to the dollar. Buy this car and together you will climb a mountain or pull a stump."

best I could do.