Jansy Mello: Perspicatious questions which, in great part, I'm unable to answer!. Your wording suggests that I'd have a thesis addressed to something that Nabokov had wanted the reader to see, or related to an intention of his to offer a "peek into his own psychology." I don't know whether Nabokov, at that stage (LATH), was trying to "communicate" with his readership, or even with himself. Personally, I doubt it. My interpretation refers to one's general tendency to distort almost all of our recollections and, therefore, to Nabokov's observable difficulty to cope  with a host of disagreable remembrances, something that, in his case, was happily allied to his genius to explore and develop his paramnesias (emotionally charged screen-memories) in different ways. 

In "Speak,Memory" the feeling of witnessing an idyllic childhood, only slightly marred by  insomnia and tedious tutors, seems to be the prevalent after-effect. Nabokov's "confessions" in LATH, though, are of a different order (the term "confessions" is inappropriate). Although Vadim's conundrum might serve as "a metaphor for his thinking about his past," ( & that's what I intended to suggest), I have no idea if it was intended as such by Nabokov and to what purpose. If it in fact "relates to the work overall" the result would be the demonstration of Vadim's relative success in evading, or feeling guilty about, his past actions (he is unable to remember them correctly and, therefore, he is now condemned to repeat them - should you accept the Freudian view*) .

However, long before Freud developped his theories about a repressed unconscious (and its layerings, like different cities or everyday objects encountered in archeological escavations) and its effects over the personality, Thomas de Quincey produced an even richer image about memory, when he compared it to a palimpsest. When I tried to retrieve his text online I came to a fascinating study that expresses almost everything I'd like to write and with greater precision. I'll be quoting only a few paragraphs, more directly related to de Quincey. I suggest that you read it in full (it's related to modern ideas about intertextuality), should the subject be of interest to you. http://lucian.uchicago.edu/blogs/mediatheory/keywords/palimpsest/. 

A palimpsest is “a parchment or other writing surface on which the original text has been effaced or partially erased, and then overwritten by another; a manuscript in which later writing has been superimposed on earlier (effaced) writing.” In other words, a palimpsest is a “multi-layered record.” [   ]In his 1845 essay titled ‘The Palimpsest’, Thomas De Quincey refers to the structure as an “involuted” phenomenon where otherwise unrelated texts are interwoven, competing with, and infiltrating each other.[   ] The palimpsest is also often likened to the human brain and to memory. Freud’s mystic writing pad is metaphor for the palimpsest and for the functioning of memory. Thomas de Quincey also writes about the human brain in a similar vein:
"What else than a natural and mighty palimpsest is the human brain? [...] Everlasting layers of ideas, images and feelings, have fallen upon your brain softly as light. Each succession has seemed to bury all that went before. And yet in reality not one has been extinguished. [...] They are not dead but sleeping [...] there is none of passion or disease that can scorch away these immortal impulses."

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* Sigmund Freud (1914). Remembering, Repeating and Working-Through (Further Recommendations on the Technique of Psycho-Analysis II). The Standard Edition of the Complete Psychological Works of Sigmund Freud, Volume XII (1911-1913): The Case of Schreber, Papers on Technique and Other Works, pp. 145-156
 
See also: http://www.magma.ca/~mfonda/freud06.html
The Psychopathology of Everyday Life. Volume VI. Translated By James Strachey. London: The Hogarth Press, 1960. Chapter 4: Childhood Memories and Screen Memories. "The indifferent memories of childhood own their existence to the mechanism of displacement: that is, they operate as substitutes in mnemic reproductions of other, significant impressions. Such significant impressions can be brought out through analysis but a resistance prevents them from being directly reproduced in the everyday situation. Types of displacement: regressive displacement: in this case the content of a screen memory belongs to earliest childhood (which has been repressed since its origination) and the mental experiences it replaces usually occurred in the subject's later life;
opposite relation: this appears more frequently than regressive displacement. In this case an indifferent impression of recent occurrence establishes itself as a screen memory but owes its existence to associations from an earlier experience which is prevented from direct reproduction by one's resistances--i.e., they are said to be displaced forward [   ]Freud leaps to the conclusion that early childhood memories are in fact revisions of such experiences. They are revisions which may have been subject to the various influences of one's psychical processes. Hence, infantile memories become, in general, to acquire the significance of screen memories. Still, Freud notes, it is vary difficult to analytically determine which memories are screens and which are not. "