not sure why your pictures are sideways, but it looks like a good vise. funny you didn't show the end shot of the missing dust cover, but i'm glad you have the other piece that is in the picture that is pinned to the vise nut.
looks like your vise just needs a good cleaning. sometimes the pinning of the vise nut is a little difficult so be careful with that part if you are not sure how to take it apart. unscrewing the little horseshoe piece in front holding the screw in can be removed and then just take the main screw out to clean, re grease and replace is a good start. PM me again if you have any specific questions or there are many, many Wilton restoration threads if you would like to see them apart and maybe some good pictures.
the freeze plug just sets in place to cover the end of the screw area so you'll see what i mean about it being flat. some of the guys have actually put a freeze plugs in a press with a round piece pressing it, but be careful not to change the diameter if you do it that way.
good luck and please post some more pictures with the cap in place if you do a freeze plug or another type.