Follow the instructions in section 6.15.
http://lammps.sandia.gov/doc/Section_howto.html#howto_15
this was also at the bottom of the vacf compute documentation page.
"Output info:
This compute calculates a global vector of length 4, which can be accessed by indices 1-4 by any command that uses global vector values from a compute as input. See this section for an overview of LAMMPS output options"