Idea for vacuuming

I'm running a 8.0 database.  I have a very large log table that is
rarely updated or deleted from.  The nightly vacuum does not know this,
and spends a lot of time on it, and all its indexes.

My RFE: When vacuuming a table, pg should try to vacuum the primary key
first.  If that results in 0 recovered entries, then assume the table
has no updates/deletes and skip the rest of that table.  I'm picking the
primary key here, but any index that indexes each row of the table will
do.  Maybe it should just pick the smallest index that indexes each row
of the table.