Ok, thanks for the clarification.
you can do this a couple ways
1. if the content of the page that is user specific is smallish (<1/3 of the page content), then u could make a generic page with the user specific parts using DT variables.
then your script step (code) could set the variables needed (if you can't do this with one of the protocol filters)
2. if the content of the page that is user specific is large >1/2 the page, then I would create a generic page that uses ONE DT variable, and that variable contains ALL the html content,
and you script step (code) would build the html content and then set the one variable used by the VSI response.
(I use approach 2)