Defender Threlfall has not featured for almost a month due to a thigh problem.
Fleming has also been missing since the goalless draw with Cheltenham in January due to an ankle problem, but the midfielder is nearing full fitness.
If both are passed fit then Jim Bentley will have a fully-fit side to choose from as the Shrimps attempt to beat the league leaders and record a third consecutive away win.
And veteran midfielder Stewart Drummond believes they can shock Micky Adams' side.
He told the club's official website: "Not many people will give us a chance going down to Port Vale but, as we proved last season, we can go down there and get a result."
- Sports & Recreation
- Jim Bentley