I can't give you a definitive answer, but based on the location of Mount Vernon on an 1864 map of St. Francis County, it appears to me that the extinct community is located in the northern city limits of present-day Forrest City.

Forrest City was established as a railroad camp after the Civil War, around which a town eventually grew and was named for Gen. Nathan Bedford Forrest. The city limits now encompass about 10 square miles, and, as I said, it looks to me like the site of old Mount Vernon is located in the northern part of the town. That's based on an 1864 map, which seems to be fairly accurate, but perhaps someone more familiar with the area can give you a definitive answer.

There's an unrelated town of Mount Vernon over in Faulkner County. It was originally known as Housville, and was renamed Mount Vernon in the 1870's.