If your using the classifieds header system meaning your not integrated with vbulletin then all you need to do is link to the server path including filename header.htm and footer.htm files which is located in your classifieds directory in your global options to load those files. You can then if you properly chmod them to 666 be able to edit the header footer files with the links in admin and place whatever html you want to load as a header and footer.
What If I'm integrated with vbulletin? And same goes for vbgallery I would like to use a custom header other than that of the home page and rest of the forums.
well no way to do that specifically in the code we load the same header as the rest of vbulletin thats part of the integration. You would have to make a code mod to use an alternate header.