Well.... Yes and no.

Is Vader, perhaps, the central character? Yes.

Would his impact be half so great if narrated by someone other than Alec Guinness? I don't think so.

George Lucas earned his place in the pantheon by recognizing, and acting upon, one simple truth that others had long failed to grasp: sound is fully one half of the cinematic experience. (please note, I'm not saying Lucas invented sound in movies, but that he fully recognized the import and impact as few others had... And moved in that direction with confidence.)

It has been reported that James Earl Jones was employed only after Lucas was dissatisfied at the tinny, perhaps even laughably so, noise coming through the mask David Prowse wore. It was a masterstroke, but one that was played off most brilliantly from Alec Guinesses' trained velvet purr and narration. (Vader does appear imposing at first, but could that sustain without Obi Wan trying to explain Vader to Luke? I, personally, don't think so...) I do not think that either Vader or Kenobi would work as well, absent the other.

Not for nothing, but voice is (in my estimation) also much the reason behind the success of both Harrison Ford and Adam Driver. Mark Hamill has come to realize this late, and has made a second career of voice work, but his initial, perceived shrill, work is, to my way of thinking, the reason his career hasn't flourished as Ford's and Driver's has. But, I do think his voice work in the sequels is perhaps the best thing about the sequels...