Interestingly enough, and something I just learned about, is that there is a deleted scene from A New Hope that mentions Vader being a Sith Lord sent by the Emperor. Obviously not conveyed in the film, but it seems Vader as just Tarkin's enforcer was not the original intention:
Oh, good grief, YouTube videos. It talks as though it's some big shocking discovery that the term "Sith Lord" was established in the original film, but it's in both the novelization and the Marvel Comics adaptation, both of which came out before the film was even in theaters. That scene is only "rare" to people who don't read. (The name Palpatine is established in the novel's prologue as well.)
Of course, what "Sith Lord" meant would not be established until the prequels. At the time, it was just an undefined sinister-sounding title like "Grand Moff." So just saying Vader was sent by the Emperor doesn't mean he was the Emperor's second-in-command; he could've been more like an enforcer, like a feudal daimyo sending a samurai to deal with a problem. Vader's helmet is blatantly inspired by a samurai kabuto, and samurai often went masked as well, so that's a credible analogy.