vacuumdb ERROR: out of memory

I'm getting error:

When I try
vacuumdb -z assessment
or
vacuumdb assessment

I get:
vacuumdb: vacuuming of database "assessment" failed: ERROR:  out of memory
DETAIL:  Failed on request of size 1073741820.

The only way i can actually analyze the DB is if i do a vacuumdb -f

The database is currently sitting at aproximatly 1/10th of my total data.

I'm on 8.3.5, SLES 11 Linux .

Any ideas?

Thanks

Dave