Translation:Marc smokes about twenty cigarettes a day.
Sorry but I have to disagree. I don't think the choice of de has anything to do with whether (une) vingtaine is singular or plural. (In passing, I think une vingtaine de cigarettes is arguably a plural noun phrase.) You wouldn't claim that de, rather than des, is used in beaucoup/trop de cigarettes because beaucoup/trop is singular, would you?
Well, I would argue that "beaucoup de" is singular. Duo consistently translates it as "A lot of," even if my personal preference is for "lots of," "A lot" is also a singular term in the auctioneering trade, even when the lot in question consists of multiple items.. ... in precisely the same way as it does for "A dozen," or "A gross," or, for that matter, "A flock" of seagulls. All singular, IMO.
If all your examples are singular in terms of the grammatical category of Number, then in English, as well as in French, the (auxiliary) verb would take the singular form in agreement with the singular subject noun phrase. Do you say, e.g., ”A dozen (of) eggs costs $ 1.00 here”? What I was trying to say in my previous comment was that though dozen and vingtaine may themselves be singular in form, the choice of de over des has little to do with the Number of the quantifier or noun that precedes it (and that in passing, the whole noun phrase une vingtaine de cigarettes will arguably be plural). According to your reasoning, when vingtaine is pluralized, say, quelques vingtaines, you would predict that the appropriate preposition that follows it is des and not de.