Depending on where you live, if you live somewhere in the US, I always liked http://www.typefrag.com/
They're really cheap and provide awesome service, allow you to use a speex server of the highest quality with no extra charge. The customer service was great, and the interface to set up the vent server and permissions online, was really easy to use. I stopped using them only when my guild disbanded about a year ago, and I didn't need my own vent anymore. If I ever needed to buy a vent server again, they'd probably be who I'd get it from.