I'd say, "don't do that". Unless you've deleted a lot of stuff and are
expecting the DB to shrink, a full vacuum shouldn't really be needed. On
a DB that big a full vacuum is just going to take a long time. If you
really are shrinking, consider structuring things so you can just drop a
table instead of vacuuming it (the drop is fairly instantaneous). If you
can't do that, consider dropping the indices, vacuuming, and recreating
the indices.
Mike Stone