To be honest I would say it is normal for the rpm to drop at least 200 rpm in drive and I have seen more. I would not be concerned. Also dont put in that Cardone. Bring it back, they have the worst parts that you can buy.
Your idle does not change with the vacuum either because it is connected to a ported source or the diaphragm is NG. Pull the hose off the distributor end and check for vacuum, you should be able to feel it with you finger. If you cant it is probably connected to a ported vacuum source. This is not always right or wrong, it depends on the application.
Artificial Intelligence is no match for Natural Stupidity