[Marco van Herwaarden]

If you have no knowledge on how to identify these IP's i strongly suggest you contact your host for support on this.

Depending on the kind of DDOS attack it would have to be mitigated at a very early stage on your server (ie. before it even reaches your webserver process, or whatever process is target of the attack) or even already need to be blocked on the outer-border routers at your hosts network.

PS Psting the exact error message you get from vB might also help in identifying the problem, although a typical DDOS attack can be done without ever even reaching vBulletin.