It depends on where you are walking. One must remember that Mount Vernon is a city with extreme variety in terms of safety, wealth, and such. I doubt that you would have any problems walking in the shopping areas, such as 4th ave, gramatan ave, fleetwood, sanford blvd and the likes. 10pm is still a relatively safe time to walk, and although there probably won't be too many people outside, there wouldn't be people waiting with a gun to shoot you or anything like that.
However, getting into the 12 and 1 am times, most businesses are closed and the entire city is pretty dead silent. if I were to see anybody walking around 1-2 am, I would think that they are either coming from a party, bar, or are up to some shady things.
