applications hang when calling 'vacuum'

> We are currently calling 'vacuum' from within our code, using the psql++
> PgDatabase ExecCommandOk() to send the SQL statement 'vacuum'.  I seems to
> work when we only have one process running, but when two processes (both
> invoking the vacuum) are running it immediately hangs.  Some postgres
> processes exist... one 'idle' and one 'VACUUM'.  The applications hang.
> I tried using the PgTransaction class to invoke the SQL but received an
> error stating that vacuum cannot be run in a transaction.
> Any ideas?
> 
> Sandy Barnes
> email    sandy.barnes@honeywell.com
>