My interpretation is in line with ender's. It struck me that Vader was purely a military leader, while Tarkin and the other Moffs were more political figures. In the Empire, the line between political and military leaders was greatly blurred, but not non-existent. Consider Vader the highest ranking general while the moffs are secretaries of the navy, army, airforce, etc with Tarkin as secretary of defense.

Vader was not a vice-emperor or a political leader in any sense. He couldn't go around killing high-ranking members of the imperial hierarchy. Vader carried out military orders. He did so ruthlessly and with a certain amount of latitude, but they were orders handed down by others (the Emperor, the Moffs).

Tarkin was clearly the leader of the decision makers (beneath the Emperor). He had the power to order the destruction of Alderaan (perhaps after conferring with the Emperor). Tarkin and Vader simply played different roles.

The fact that Vader's role was somewhat limited does not at all diminish his character. Throughout history, great military leaders have had to answer to those who govern. After the Senate was dissolved, after Tarkin was killed, it certainly created a vacuum for Vader to fill and exercise even greater power.

That Vader should embody pure evil and total power is one of my SW pet peeves (a thread topic I've been meaning to make). Vader was the main villain in SW, but the universe was richer and more complex than it might seem at first glance.