Having lived in a lot of US cities, I've found it goes :
Proximity to a college / Age of residents > City density > City Population
There are some very dense, even walkable cities, that have less vegan options than the small town with a college that I lived in. Someone mentioned DC, but I found it had even less vegan options than a college town in a red state I visited. The worst of course is non-walkable cities (which is like 98% of the US), and rural areas.