Nick Grundy has challenged me to find a good example of Gratuitous Virtuosity in VN's work. In general, I am not persuaded that GV is a sin (and even sometimes a mitzvah---GV, GV!) but, if pushed, I would nominate ADA, I-26 which explicates the code used by Van & Ada during their 1884-88 separation. The three-page chapter would seem to be all in aid of permitting the reader to decode a short, inconsequential phrase in the preceding chapter:

Van plunged into the dense undergrowth. He wore a silk shirt, a velvet jacket, black breeches, riding boots with star spurs — and this attire was hardly convenient for making klv zdB AoyvBno wkh gwzxm dqg kzwAAqvo a gwttp vq wjfhm Ada in a natural bower of aspens; xliC mujzikml..... .

The "decrypt" (as we ex-cryptanalysists racily put it): is "[making] his way through the brush and crossing the brook to reach Ada"......., they embraced." At the end of I-26 Ada herself suggests "omitting this little chapter altogether." Boyd in the Cyberedition of his book on ADA points out that the coded passage calls attention to surrounding textual allusions to Marvell and Rimbaud poems. Yes, but such would be the case even if I-26 is omitted. Nor can the chapter can be justified on structural grounds---nothing in the book would be affected if the chapter were not there. In closing, I remark my opening comment that GV is not necessarily a mortal sin. A close look at Ada shows that there are other "gratuitous" chapters.