Nabokov can tear your brain apart with narrative. In nearly all of his works, and especially in Lolita and Pale Fire, he invites the reader to examine every word as a piece of the narrator, always insisting, "This is not me, and if you think it is, you're a dolt." What, then, are his determined doters supposed to think when finally confronted with Vadim Vadimovich, emigre-novelist, almost self-aware deranged fictional character, and butterfly-hater? God only knows.
Obviously, he's not Vladimir Vladimirovich. He's something else. Maybe he is meant to be an inevitable distortion of Nabokov, but even that's questionable, as is everything in Nabokov's fiction.
Here's a thought. Perhaps, as is (almost) evident in Transparent Things, Nabokov eventually became so intrigued by the idea of networks of perspectives in fiction (the perspective of the narrator interacting with that of the reader, and maybe just a tittle of his own), that he couldn't resist the idea of writing a novel from the perspective of a fictional fictional Nabokov.
All fiction can be compared to the reflection of a painting in a puddle. Nabokov teaches us that the aesthetics of the puddle's ripples, manipulated by the right hands, can be as (nay, more!) breathtaking than those of the picture itself.