vacuum in Postgresql 8.0.x slowing down the database

Hey all,
   I had posted sometime back asking for the best way to perform vacuum
with a lower priority - I did tune it up to a lower priority
and still noticed that the other database queries are slowing down
with a vacuum on one big table. I also tried to upgrade Postgresql to
8.0.15 as suggested and I still could reproduce the problem.

It happens when I try to vacuum one particular table which has 9.5
million rows and all the other inserts/selects are slowing down by a
factor of 10 times. I am using
vacuum_cost_delay = 100
vacuum_cost_limit = 200

Even if I cancel the vacuum operation or let the vacuum complete - the
slowdown continues to persist till I restart my application.

Whats the best way to analyze the reason Postgresql is slowing down? I
had a look at pg_locks (did not know what to look for) and also tried
starting postgresql with the debug option using: postmaster -d 5
-D /var/pgdata (saw a lot of output including the query being performed
but could not gather anything useful)

The big table has never been reindexed and has a primary, unique key
with btree index and one foreign key constraint.

--
Vinu