I want to render a smarty template without closing page, so I can include smarty (xcart) content in vbulletin pages on same domain

I have been going crazy for hours trying to pull part of a smarty (xcart) site into another page on the same domain.  I've got xcart and vbulletin on there, and the xcart folks integrated the login systems.  This all works reasonably well, but I've got my page header (nav, login box) in an xcart template and want to pull it into the top of the vbulletin pages.

I found out how to include PHP files in vbulletin templates, and tested with a simple "echo 'hello world';" - worked great.

Next I made a home2.php in xcart that only pulls in the header - works fine when I view it directly, ie www.domain.com/xcart/home2.php.

I included the home2.php in a vbulletin page, and it works, but all content after the home2.php stuff is cut off.  I suspect that smarty's rendering closes out the page, especially since I never seem to see smarty's "func_display" called more than once in a row.

Anyone know whether smarty does indeed close out the page, and maybe a way to just load the HTML result of func_display into a variable to display inline with other stuff?

I can't have this part static - it must have the functioning minicart.  I can't call the page using anythign like file_get_contents because the request needs to come from the client's machine to proprerly show login status etc.

Basically I'm looking for a version of $smarty->display that will just spit out HTML without sending any headers or closing out the page or anything...