It probably needs a vise nut. I used to stock them on my tool truck. Examine the threads closely inside the nut. The screw threads rarely wear out enough to cause an issue.
The "main vise nut" (#3 in the diagram) looked good when I removed it, and the spindle screwed into, and out of, it just fine. Or looking at this another way, it's like a nut in that I can spin it on, and off, the spindle w/o any problems.
When the vise was on the bench it was nice and smooth, too. The spindle would back-out of the vise slicker than snot, but it didn't seem to be connected to the "front jaw" (#1 in that same diagram) in any way, and I would have to physically pull on the front jaw to get it to open.