This occurs because the vice is not being used correctly.
Solid work-pieces that completely cover the clamping surface of the jaw provide the best clamping performance. If the work-piece is small and needs to be clamped at a corner of the jaw, there is a possibility that the part will not be securely clamped when the vice is new. This is due to the close lateral tolerances in the mounting of the movable jaw assembly when new.
If it is necessary to machine this type of parts, the best alternative is to place a similar part on the opposite side of the jaw.
