vacuum

Hi all,

I'm sure many other people have faced this and I'm wondering what solutions
were found.

We have a product that uses PostgreSQL as its database. Currently, via a
web-based administration interface, we provide the administrators the
ability to vacuum their database whenever they wish to. We would prefer to
provide this functionality automatically such that they don't need to think
about it.
We could easily have the system vacuum once a day (or week or whatever) but
we are concerned that the vacuum process will slow down the system while it
is being performed. This means that we would need to try to do some analysis
on the business of the system and then vacuum at a perceived slow time. This
sounds difficult.
What do other people do? How much of a hit is vacuum when the system is
operating? Any other ideas?

Thanks in advance,

--Rainer