VACUUM Question

Hi,
just a few questions on the Vaccum

I run a vacuum analyze on the database every night as part of a
maintenance job.

During the day I have a job that loads 30-70,000 records into two tables
(each 30-70k).
This job runs 2-3 times a day; the first time mainly inserts, the 2nd,
3rd time mostly updates.
Both tables have in the area of 1-3Mio records

How reasonable is it to run a Vacuum Analyze before and after the
insert/update of the data.

Also, I noticed that i get quite some performance improvement if I run a
count(*) on the two tables before the insert. Any reasons for that?

One more question; on one server the Vacuum Analyze before the insert
takes approx. 2min after that the same command takes 15min.

I run PG7.3.4

Thanks for any help

Alex