Native Detroiter here. Vernor's is not distributed in NYC, but can occasionally be found in places like Nosher mentions because someone brings it in themselves. The closest places Vernor's is officially distributed are, I believe, in western New York state and in central Pennsylvania.
VERNOR'S FOUND IN MANHATTAN! This ex-Detroiter stumbled on Vernor's at Kitchen, a good Mexican takeout place at 218 Eighth Ave between 21st and 22nd. Please go and buy enough to encourage them to keep carrying it--but please leave me a couple of cans.