Dunlap was an economy brand of Sears that replaced the Companion brand sometime around 1940. It was a less expensive alternative to the better quality Craftsman label. Dunlap tools were generally of similar quality as Craftsman, but made with slightly lesser materials or manufacturing processes, or a simplified design. that allowed them to be made and sold cheaper. By today's standards, they are well built.
Without seeing a picture of the condition of the vise I couldn't comment on the price, but it seems a little high unless it's in pristine condition.