Problem is, we run this on a DSl line with max of 1.5megs upload. That is roughly 180kbyte/sec upload for us.
We average about 50 ppl online during peak time, rather more (70 this sunday). Each user taking about 2-3 Kbytes and the line is already in good use. If 15 people raid they use more than 2-3 kbytes/sec. These are just estimates.
Bottom line is, this doesn't leave us lots of bandwith for a Vent Server (or Teamspeak). So 15+ people using Vent and 70 people online = Lag.
Still looking into other possibilites like IRC or a Shoutbox.