I think Jim is right that a tcl based solution will most likely prove fast enough. There are a number of programming decisions you need to make if you choose his path for a roll-your-own vhost server. I have several tcl only solutions, one that is a proxy and one that uses url mapping. The mapper is at http://zmbh.com/vat/ and the proxy is at http://zmbh.com/tclvhr/ I don't know if either solve your particular problem, but they are written in tcl and easy to configure or adapt to your situation. Bug me if you have questions.
--Tom Jackson
