I saw a really neat idea in a 1996 ME article by H D Bickley
He needed a very small vice so he could bend up a small bracket that would have fouled the vice jaws, it wasn't worth making a block to shape it round for a 1-off.
His idea was to grip a pliers in the vice, pointing upwards, so that the vice jaws held it shut.
Very neat and I can think of several times in the past when this would have saved me much grief – probably worth making a small pair of 'slave jaws' that work in the same way.
Neil