When to Vacuum

I posted this on 1/20 but never got delivered....trying again....

I was wondering if I can use any of the pg_stat* related tables to
determine if I
need to vacuum or not.

More specifically, instead of assuming a period (based on one's usage),
if I was
to vacuum more adaptively, I could have a cron job to check certain
statistics table
every 10 minutes (or hour) and if a threashold is reached a vauum be
performed.

If this is possible, please specifiy the condition for vacuum and vacuum
full please.

Thanks