You should see the DB shrink in about 24 hours or so. You can get further reductions by changing "Days to Retain Raw Performance Data" to a smaller number (default is 14 days).
You could also do a manual vacuum on the DB as well (link to Postgres instructions below). Note if you decide to do a FULL vacuum, then you need to shut down the VMan service first.
Let me know if you need additional information.
Brian, thanks for your response. I am seeing the reduction that I expected. And thanks for the VACUUM steps as well.