machine vice back to life

Take a sealable metal container (that will accept the vise) and place the vise inside.
Cover with Brake Cleaner and let sit for a day or 3, externally agitating [swirling the container] whenever you think of it.
Remove the vise and let it dry in a well-vented area.
Knock off as much rust from the screw threads as possible with a soft brass brush.
Return the vise to the Brake Cleaner for an hour or two, then dry it again.
Apply a good penetrating oil to all threaded surfaces and let sit for a while, then try loosening. [Use more if required.]
Disassemble and repeat Brake Cleaner to remove oil and rust within cavities.
Treat parts with a good rust-killer and reassemble.
Worked for me.

an old automotive mechanic trick to free rusted stuck parts is to use coca cola - the acid and fizz seem to do the trick

For sure I'd try electrolytic derusting first. 5 gallon bucket, 1/4 cup washing soda or baking soda, clean water, 1 or 2 steel electrodes (I use railroad track tie plates with copper wire soft soldered to them) and a wire to your part. Use a car battery charger and hook your part to the BLACK (-) wire, and the electrodes to the RED (+) wire and let it run for 1-3 days, then take it out and scrub it with a stiff brush under hot water and see how you're doing. Might need to disassemble it at that point and try again. It works real well. I've got a bucket sizzling downstairs right now cleaning old pipe threading die heads.
GWE

This reminds me of an idea I had ages ago. I wanted to make a metal sphere for a Van de Graaff generator and electroplate it with copper. I was going to make it from two propane cylinder bases cut off and welded together. My welding set has a 75 amp battery charger incorporated into it, and I wondered if this would be good for electroplating? Anyone ever tried it? By the way, the Van de Graaff project is "on hold" for the time being, so I'm just asking out of curiousity.
Chris

I'd use the salt & vinegar mix on it. Or, some good spray oil (Break-away has been my favorite for some years now) and judicious hammering.
John