Size for vacuum_mem

Trying to improve my vacuum times. Currently have vacuum_mem set to 128MB.

What is my best bet to improve vacuum time? Increasing this number?

Have 1GB of ram in the machine.
Some of my settings:
wal_buffers = 256          # min 4
shared_buffers = 12288     #96 MB (buffers * 8K)
sort_mem    = 131072   # 128 MB measured in K
vacuum_mem  = 131072   # 128 MB measured in K
wal_files = 10                  # range 0-64
checkpoint_segments = 12        # in logfile segments (16MB each), min 1
checkpoint_timeout = 900  # in seconds, range 30-3600

Currently a vacuum full takes 3+ hours and very soon the amount of data
will increase. Have a very high turn over of data because this is a copy
of data on a different system. Because of that I every day delete/merge
entire range of dates deleting/adding million of records.