Vaccum C.B. working principle is that Arc quenching medium 
is air our vaccum when fault occur then C.B.works because 
of arc making by vaccum

normally the contacts of CB are closed,when a fault occurs
these contacs are pulled apart hence an arc will be formed,
this arc has to be quenched for successfull
operation.circuit breakers are classified in different
ways,one of the ways is according to the arc quenching
medium.in air circuit breaker the arc quenching medium is
air,and in vaccum circcuit breaker it is vaccum