Darth Vader is not in command of the Death Star; Grand Moff Tarkin is, and the decision of what to do in this kind of situation is Tarkin's not Vader's. The appearance of Kenobi on board the station (in the very same ship that "blasted its way out of Mos Eisley"—which they may already be planning to release and track) is an important and unexpected event, which means the overall commander needs to be notified, which is just what Vader does.
Tarkin appears to have complete confidence in Vader's ability to deal with Kenobi; Jedi matters are obvious within Vader's bailiwick. So Vader informs his superior of what needs to done to deal with the Kenobi situation, knowing that Tarkin will accept his evaluation of what is needed. However, had he not brought the matter to Tarkin's attention at all, that would have been going to far in exerting Vader's decision-making autonomy.
Finally, remember that Darth Vader is confident, when he discusses the situation with Grand Moff Tarkin, that "escape is not [Kenobi's] plan." Presumably, Vader has also inferred this information through his use of the Force. However, if Vader knows this, it means that there is no need to hurry the confrontation with his old master. The two will meet soon enough, when the time is right, and presumably both participants are well aware that the duel is going to happen. Regardless of what happens to the rest of the Millennium Falcon's crew (plus the princess), Vader knows that Kenobi is not getting away.