"Sorry too many clients" error and vacuum command

1. Postgres in default does not delete the expired/unnecessary lines from
the tables, therefore we have to run vacuum regularly to delete these lines
in order to prevent the database from becoming too large. Is there any way
to change the default, so that the updated lines do not duplicate and the
unnecessary lines are deleted completly from the table ? Actually can we
forget about vacuum completely?
2. The vacuum runs for hours in our database server, sometimes it even kills
Postgres, we do not know why. Is there any way to implement any other
method/command instead of using vacuum or, if not, how can we speed up this
process?
3. What can be the reason for the error message "Sorry, too many clients
already!" Can we avoid this error in any other way than switching the number
of backends with -N and -B to a higher level?

If anybody can help with these problems we would be willing to pay for the
help as we have run out of ideas,

thanks for any help,

Gyula Toth