by vents i guess you mean the ends of the vents that you can shoot to pieces ...
( or if you want space where you move then just construct out of brushes )
you need them to be breakable so make your brush put a texture on it that looks like a vent ( can be found in cstrike or halflife.wad )
then press Ctrl + T and selct " func_breakable "
in the "strength " area write a value ( the bigger the value the more you have to soot it to break )
and in the " material type " area select " metal "
thats about it
oh and if you have a vent-texture that has blue parts in it then also set this :
" Render Mode " to "solid"
and " FX amount " to "255"
this will make the blue part invisible so you can see trough it
Wtf is that ? its a bird ! no its a plane ! amm maybe its a plastic bag . NO ! its SUPERJER
! the greatest super hero ever!