Very simple to initiate an AJAX request. vBulletin uses the YUI JavaScript framework.
Code:
YAHOO.util.Connect.asyncRequest('POST', scriptpath + '?do=ajax', {
success: this.handle_ajax_response,
failure: this.handle_ajax_error,
timeout: vB_Default_Timeout,
scope: this
}, SESSIONURL + 'securitytoken=' + SECURITYTOKEN + '&foo=' + foo);
@Brooks: Links with search IDs won't work after vBulletin cleans up.