This is a list of files that need to be removed to make your installation a forum only install, without the suite products.
You can remove the list of files before, or after an install. If you decide to remove the files after installing/upgrading you'll need to disable/uninstall the products as well in the AdminCP > Products & Plugins > From the drop down Uninstall/Disable.

You will need to change the index.php file as well, find:
PHP Code:
/**
 * Use the CMS as the default script:
 */

require('content.php');


/**
 * Use the forum as the default script:
 */

// require('forum.php'); 
Change it to:
PHP Code:
/**
 * Use the CMS as the default script:
 */

// require('content.php');


/**
 * Use the forum as the default script:
 */

require('forum.php'); 

The file locations are given based on the vBulletin zip file as provided in the members area:
  • upload/
    • blog.php
    • blog_ajax.php
    • blog_callback.php
    • blog_external.php
    • blog_inlinemod.php
    • blog_post.php
    • blog_report.php
    • blog_search.php
    • blog_subscription.php
    • blog_tag.php
    • blog_usercp.php
    • content.php
    • list.php
      • admincp/
        • blog_admin.php
        • cms_admin.php
        • cms_permissions.php
        • cms_content_admin.php
      • clientscript/
        • blog_ajax_calendar.js
        • blog_ajax_latest.js
        • blog_ajax_rate.js
        • blog_ajax_tagsugg.js
        • blog_dragdrop.js
        • blog_grouplist.js
        • blog_inlinemod.js
        • blog_quick_edit.js
        • blog_quick_edit_entry.js
        • blog_quick_load.js
        • blog_quick_load_entry.js
        • cms_textedit.js
        • vbulletin_cms.js
        • vbulletin_cpcms_layout.js
        • vbulletin_overlay_test.js
        • yui/
          • grids.css
      • includes/
        • adminfunctions_cms.php
        • class_dm_cms_layout.php
        • class_dm_cms_widget.php
        • functions_cms_layout.php
        • blog_adminfunctions.php
        • blog_functions.php
        • blog_functions_category.php
        • blog_functions_log_error.php
        • blog_functions_main.php
        • blog_functions_online.php
        • blog_functions_post.php
        • blog_functions_search.php
        • blog_functions_shared.php
        • blog_functions_tag.php
        • blog_functions_usercp.php
        • blog_init.php
        • blog_plugin_useradmin.php
        • blog_plugin_usermerge.php
        • class_bbcode_blog.php
        • class_blog_entry.php
        • class_blog_response.php
        • class_blog_search.php
        • class_dm_blog.php
        • class_dm_blog_category.php
        • class_dm_blog_custom_block.php
        • class_dm_blog_rate.php
        • class_dm_blog_trackback.php
        • class_dm_blog_user.php
        • class_dm_deletionlog_blog.php
        • class_profileblock_blog.php
        • class_reportitem_blog.php
        • class_trackback.php
        • class_usercss_blog.php
        • class_xmlrpc.php
        • class_xmlrpc_pingback.php
        • version_vbblog.php
        • cron/
          • blog_cleanup.php
          • blog_dailycleanup.php
          • blog_pending.php
          • blog_views.php
        • xml/
          • product-vbblog.xml
          • product-vbcms.xml
          • cpnav_vbcms.xml
          • cpnav_vbblog.xml
          • cssrollup_vbblog.xml
          • cssrollup_vbcms.xml
          • hooks_vbcms.xml
          • hooks_vbblog.xml
      • packages/
        • vbcms/
        • vbblog/
Key:
Folder: name/
Files: name.extension in its folder
If only a folder is listed, delete its contents (eg: packages/vbcms/)


If you wish to add them back, reupload the files and run install/upgrade.php and enable/disable the plugins if necessary.

Attached is a unformated list of files and their folders.