EDNOTE. NABOKV-L thanks Nick Grundy for these examples and comments. They nicely illustrate
the complexity of the problem. There are many different categories of parenthetical comments--some more interesting than the "dictionary meanings" of "parenthetical."  If we want to establish what is unique  or atypical about VN's usage, we must first  set up a a set of categories and compare their distribution in VN's usa vs that of other writers.
 
----- Original Message -----
From: Nick Grundy
 

The "extra" chapter to 'Speak, Memory' ('Conclusive Evidence' as was) offers:

"The solution of the riddle theme is also the solution of the theme of exile, of the intrinsic loss running through the whole book, and these lines blend, in their turn, with the culmination of the rainbow theme (“a spiral of life in an agate”), and merge, at a most satisfying rond-point, with the many garden paths and park walks and forest trails meandering through the book."

Mona induces one or two in 'Lolita': "It was characteristic of Lo that she chose for her closest chum that elegant, cold, lascivious, experienced young female whom I once heard (misheard, Lo swore) cheerfully say in the hallway to Lo - who had remarked that her (Lo's) sweater was of virgin wool: 'The only thing about you that is, kiddo....'"

The Reverend Rigger on the facing page of my copy: "...beautiful Mona replied: 'Well, sir, the fact is Dolly is not much concerned with mere boys. Fact is, we are rivals. She and I have a crush on the Reverend Rigger.'  (This was a joke - I have already mentioned that gloomy giant of a man, with the jaw of a horse: he was to bore me to near murder with his impressions of Switzerland at a tea party for parents that I am unable to place correctly in terms of time.)"

Kinbote, in 'Pale Fire':

"Let me state that without my notes Shade’s text simply has no human reality at all since the human reality of such a poem as his (being too skittish and reticent for an autobiographical work), with the omission of many pithy lines carelessly rejected by him, has to depend entirely on the reality of its author and his surroundings, attachments and so forth, a reality that only my notes can provide."

Pnin:
"...Of course, Victor's mother was not really dead; she had left his everyday father, Dr Eric Wind (now in South America), and was to be married in Buffalo to a man named Church."

Kinbote, 'narrating' a conversation with Shade, uses them to remind the reader that his memory is as good as verbatim:
"He said that ... (much exploited by Left-Wingers) ... (he admitted) .... (he thought) ... (tensing of simian nostrils, sickening dulling of eyes) ... (observed Shade) ... (he agreed) ... (he conceded) ... (he continued) ... [he said] ..."

Humbert again:
"Now, in perusing what follows, the reader should bear in mind not only the general circuit as adumbrated above, with its many side-trips and tourist traps, secondary circles and skittish deviations, but also the fact that far from being an indolent partie de plaisir, our tour was a hard, twisted, teleological growth, whose sole raison d'etre (these French cliches are symptomatic) was to keep my companion in passable humour from kiss to kiss."

"We passed and re-passed through the whole gamut of American roadside restaurants from the lowly Eat with its deer head (dark trace of a long tear at inner canthus), 'humorous' picture post cards [...]"

"I asked Mr. Potts was he quite sure my wife had not telephoned, and what about that cot? He answered that she had not (she was dead, of course) and the cot would be installed tomorrow if we decided to stay on."

"(what shadow of us was he after?)"

"(she was a very mediocre mermaid)"

"In the afternoon, Haze (common-sensical shoes, tailor-made dress) said she was driving downtown to buy a present for a friend of a friend of hers [...]"

"The sport was so excellent, its results - in my case - so ruddy that I stayed on for a whole month after I was quite well (sleeping admirable and eating like a schoolgirl)."

And here, to close, are a pair of triplets from Pnin:
"In order not to complicate the modish triangle of Freudian romance (father, mother, child), Liza's first husband had never been mentioned."

"He was a well-known, frankly academic painter, whose soulful oils - 'Mother Volga', 'Three Old Friends' (lag, nag, dog), 'April Glade', and so forth - still graced a museum in Moscow."

That last one reads rather like the Russian wordplay I can't quite remember about the word "God".

There are also a number of instances in Pnin where parentheses enclose first translations of Russian, then the Russian itself after the translation, and interestingly Kinbote uses the same technique in 'Pale Fire' with Zemblan.  However, only a few of the ones I've turned out above actually fit the original bill, so enough of that.

Nick.