The news of this plan broke, as far as I can tell, at io9, which features an NVIC-created image of their antivaccine message taped to Kit Kat bars, ready for distribution to unsuspecting and probably already vaccinated children on Halloween. I'm not sure how many children the NVIC folk have been around, but if they're imagining that the wee little tots in their Frozen costumes and ninja duds are going to select a Kit Kat, carefully examine the wrapper for vaccine-related messages, and then carefully preserved said message for their parents -- well, that scenario is about as accurate as the NVIC name.
The one reader here who might have read The Moonstone would recognize that this NVIC attempt to proselytize the unsuspecting stranger is redolent of the Victorian practice of handing out evangelical tracts to sinners, each tract containing a cautionary tale deemed suitable for the sinner's transgression (sample tract described in The Moonstone: "A Word with You on Your Cap Ribbons"). The tracts in the novel ended up discarded, unread. I have almost no doubt that any related missives adhered to the wrapper of a chocolate bar will meet the same fate. And they're too late anyway. If you're old enough to trick-or-treat and eat Kit Kats, odds are that you're well into your childhood vaccine schedule.
Meanwhile, I'm wondering if the anti-GMO crowd is now considering putting a dueling message on the Kit Kats, given the candy's reputation for being a gateway food to a Monsanto addiction.