This is the clutch slave cylinder that is located on the transmission. When the slave cylinder goes bad usually the master cylinder (251721401A) isn't far behind.
Diesel Vanagon clutch kit.
Bellows for 5-speed Shifter from 9/85-1991
Gear Shift Bellows Intermediate Piece 1.6l TD 8/84-7/92 Vanagon
Vacuum Acutator for Syncro Vanagon Locking Differential