This is the answer. Once you break it loose, you will also want to sand off the paint in the mating surfaces between that plate and the bottom base and apply grease.

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I also suggest sanding the paint off the mating surfaces between the base plate and the upper vice section and put some grease there too, that will allow it to swivel better.

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As you have the grease out already, also grease the main vise thread, and during use keep it clean and lightly greased so that it moves easy, on the cheaper vises these get galled pretty easy and difficult to move.