Although it was counterproductive for NYC Council to ban the sale of e-cigs, smokeless and cigars to anyone under 21 (as all of these products are far less hazardous than cigarettes), 18-20 year old smokers and vapers in NYC will still be able to easily
buy e-cig products.
They'll be able to do so either by driving to adjacent counties in NY or to CT (or to NJ for 19-20 year olds, as NJ's minimum age for e-cigs is 19), or by purchasing e-cigs from online vendors located outside NYC (as NYC has no legal authority to ban online sales of anything, and NYC cannot enforce online purchases because it cannot inspect any (let alone all) UPS, FedEx, Postal Service and/or other packages sent to NYC residents to see if they contain e-cigs or
tobacco products).
The only e-cig vendors who will need to comply with NYC's age 21 law are brick & mortar (and probably online) vendors that are located in NYC, as NYC Health Dept is likely to conduct sting operations against brick and mortar stores (and perhaps online vendors) in NYC to enforce the age 21 law.