Cheating how? & Cheating whom? are two questions one could ask in return to understand where the other person is coming from.
I don't think it's challenging to understand what it is possibly meaning.
Cheating because we (vapers) still get to engage in act of smoking/using nicotine while claiming we quit.
Cheating yourself foremost, but also cheating the social construct we have built around 'quitting smoking' as in I don't do anything at all that resembles that activity anymore. Like if alcoholic was saying, I quit drinking, but was found in bar everyday drinking near beer. We'd perhaps think they are cheating themselves and cheating/lying to others if they claim they are free of all the things that went into their addictive behavior. Not a great analogy cause near beer isn't understood as a tool for cessation like vaping/NRT's are.
Personally, I think (some) people are too hung up on quitting, and boasting about it. But I also very much understand the liberation factor that comes from stopping something that was intrinsic to your self identity and no longer feels like a weight pulling you down.