Doble, i'm hardly a expert on this topic but if I understand what you're saying that there's a denial of service attack on a specific IP address through your switch possibly overloading that switch, you need to null route that IP.
You should speak with your provider, basically get some professional help to deal with the problem immediately.
A VLAN doesn't really have anything to do with security. It has it's own benefits, if you created it on your own switch and your switch supports it, yes you can limit traffic to that VLAN's port to 10mbit, 100mbit, or possibly 1mbit increments, etc. However as I pointed out, if you have an attack using a large amount of bandwidth directed at an IP being routed to that switch, the switch has to process and deal with that.
You need to null route that IP, and get help to deal with the immediate threat so you're not attacked attacked again as soon as you setup a different IP or VLAN for the server.
Since you only mentioned your switch, bandwidth and IP, I've only been able to assume you've experienced a DoS attack at this point.
If it's web/httpd related would start hardening your Apache install, or OS. If it's directed at another service, research that particular topic for more help. There's even a number of useful posts on this forum for apache and security options using open source modules.
Depending on the severity and your budget you can look at hardware solutions placed between your switch and your connection to your bandwidth provider. Or switch to a provider that is better equipped to deal with such a problem and has more bandwidth available to handle the incoming traffic.
I just asked a question about VLAN's and bandwidth traffic limits through switches in another topic - but since you mention both those things here - may I ask you, what switch (make/model) would you recommend as being good all around for controlling a few servers (12?) in a rack for individual shared colo clients? I'm thinking of getting a rack and offering the excess space but I want to be able to obviously either limit and/or keep close watch on the bandwidth usage. Also, if VLAN's are a function of the router (?) - then I'd like to have each client's servers on their own VLAN. So, ease of setting up VLAN's is a factor on deciding which switch to purchase. Port speeds I guess 10/100Mbps - my guess is 1000Mbps would be overkill and way too expensive.