VACUUMing a live site OK?

Hi,

I have a web application using PostgreSQL 7.1.3 with a table that
experiences alot of UPDATEs. I shouldn't be updating the data so much to
start off with but can no longer avoid the situation.

I would like to vacuum a table (1 table only) to maintain an acceptable
performance. Is it safe to vacuum a table when users are attempting to
SELECT from or UPDATE to it? I would imagine that the table would get locked
and there should be no problems.

Any help is greatly appreciated. Thank you very much in advance.