Mavericks said:

Ok, but still any interrogation is the duty of army, its intelligence, not the President of even the Defense Minister (who equivalent Vader used to be to). He ordered to bring them to him personally.

The record shows that for the one interrogation shown in the entire trilogy, Vader was not only involved but central. Different armies are organized differently, especially when you've got force users who may have special abilities regarding interrogation (sensing conflict in others, thinking about sisters, etc). There's no doubt at all that whatever Vader's official rank, he's involved in interrogations.