I spent 23 years of working Spring and Fall National Championships at the North South Shooters Association between 1974 and 2005 (when the Marine Corps did not have me in other parts of the world). Getting rusted nipples out of Muskets and Revolvers was a VERY common thing I did at each shoot.
I could not agree with Zonie more that one ABSOLUTELY NEEDS a pair of "pieces" or "lengths" of brass rod to place in opposing cylinder holes and that is what you tighten in the vise - to hold the cylinder securely and get stuck nipples out.
DO NOT try to pad the cylinder and hold it in the vise. It either won't hold and then slip loose or you will crush/damage the cylinder.
I just went to the shop and measured the two pair of brass rods that I have used for every kind of revolver cylinder made in .36 and 44 cal. in all those years. 3/8" Brass Rod is perfect for .36 caliber revolver cylinder holes and you just cut them to length from that size rod, but that diameter is too small for .44 caliber cylinders. I took 1/2 inch brass rod and turned the ends to .440 inch on a lathe and never had a problem with them fitting any .44 cal. cylinder. However, most folks don't own a lathe and you don't have to go to that much precision for personal use.
A 7/16 inch diameter rod is .4375" and just .0025" (two and a half thousandths of an inch) under the .440" size I use. What that means is you use a 7/16 inch rod and don't have to worry about turning it down on a lathe to fit .44 Cal. revolver cylinder holes. This diameter of rod will hold the cylinder better and it won't twist as much on you as with a 3/8" diameter. You just cut off a couple of pieces. I made the lengths of my brass rod 5 inches long to make it more handy, but as Zonie mentioned, you can get away with pieces at least 3 inches long.
Perhaps the easiest way to get two lengths of brass rod is to call a machine shop near you and see if they will cut a 7/16" brass rod into two 4 or 5 inch long pieces and sell them to you. If that doesn't work, you can usually get Brass 7/16" bolts from Marine Boat Shops like West Marine or other shops. As Zonie mentioned, just make sure there is at least 3" of bolt below the head. I would advise cutting the heads off the bolts with a hack saw.
In a pinch, I have used pieces of 7/16" hardwood ramrods to hold .44 Cal. cylinders in vises, though they are not strong enough to remove ALL stuck nipples in cylinders. Still, they are handy to have when doing normal nipple removal/replacement for normal cleaning. If every revolver shooter had two pieces of hardwood ramrod (sized to fit his/her cylinder) to hold the cylinder in a vise and occasionally remove and grease the threads of the nipples, there would be a LOT less rusted/stuck nipples that gunsmiths have to get out.
If the nipples won't come loose by using the hardwood or brass rods to hold the cylinder, then as others mentioned you have to soak the cylinder in penetrating oil. The easiest way to do that is cut the top off an aluminum soda pop or beer can as the cylinder fits nicely down inside the can. Put the cylinder down in the can nipple end first and that leaves the chamber holes upward. Fill each cylinder with oil a little above the front end of the nipples and also around the cylinder to that height. Let it soak overnight. Save the oil by pouring it into another container, get your rods out to hold the cylinder in the vise and now see if the nipples come loose. If not, then put the cylinder back in the can, pour the oil back in and soak for another day and try again. I think the longest I ever had to soak a cylinder in Kano's Kroil Oil was three or four days.
If you are not familiar with Kroil Oil, you can get it at almost every auto parts store. An 8 oz. can is all you need. Here is a link:
http://www.kanolabs.com/
I have also used a mixture Tranny Fluid and Acetone as the penetrating oil. BUT, if you don't normally have them at home separately, the Kroil is usually easier to buy and use.
I would very much advise AGAINST using a Propane or other Torch to heat the nipples to loosen them for most people. WAY TOO EASY to heat the cylinder too much, unless you are good with such a torch. A heat gun is the most I would advise trying. Still, it is better to soak the cylinder in oil and keep trying it every day until the nipples come loose and there is almost no possibility of damage to the cylinder that way when using the hardwood or brass rods.
For most people, if you get the really RARE cylinder with rusted stuck nipples that won't come out by soaking with penetrating oil for days and using these techniques, then my advice is to take it to a gunsmith or machinist who is experienced with this work.
Gus