I assume you mean that as it rotates there is a point at which it feels stiffer, or is harder to turn? Depending on the type of vise and the cause of the problem, it may not necessarily be a matter of lubrication. Most vises have some sort of provision (usually a plunger with a spring & a screw) for applying friction to the rotating mechanism, so that it isn't completely free spinning. Sometimes the plunger drags and causes this situation. I have a vise that I kept fussing with the bearing until I figured out that wasn't the problem.
I'd recommend a thorough disassembly & cleaning to check if anything is wearing unevenly or has crud trapped somewhere. Then first put it back together without any lube and see if the problem still persists. It may be one of those things that fixes itself just thru the process of taking it apart and putting back together...
My GRS vise is from the 90's and the lubricating oil in it had gotten thick over the years. I've cleaned it and reoiled a couple times using 30W automotive oil. The next time I'm going to try something synthetic to see if it lasts longer.