Howard Kazanjian on page 64 of the book "The Making of The Return of the Jedi" (Del Rey, 1983):

"Mark Hamill is coming back for one day of shooting later this month [November 1982] to do the scene where he builds his new lasersword..."

This scene was a re-shot made after the principal shooting was over. It was shot the same day as the scene with Luke on the X-Wing cockpit talking to Han and Leia via intercom, to sustitute the cut sandstorm scene.

It is possible the shots of Vader are recycled because they didn't bother to shot new Vader scenes so late after the main shooting was over.