The code may lead you to believe you have an incompatible template but if you are not trying to use an old template currently that is not the issue. I actually deleted all 3.x style templates to make sure.
What the issue is, is old plugins that are not compatible but Vbulletin does not seem to account for this except that you'll see a fatal PHP error. You should disable all plugins and then enable one by one until you find the one that is causing the issue.
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP Stack trace:
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP 1. {main}() /www/vhosts/forums.int.com/forums/index.php:0
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP 2. require() /www/vhosts/forums.int.com/forums/index.php:43
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP 3. require_once() /www/vhosts/forums.int.com/forums/forum.php:67
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP 4. eval() /www/vhosts/forums.int.com/forums/global.php:29
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP 5. fetch_template() /www/vhosts/forums.int.com/forums/global.php(29) : eval()'d code:197
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP 6. vB_Bootstrap->process_templates() /www/vhosts/forums.int.com/forums/includes/functions.php:4208
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP Fatal error: Call to a member function query_first() on null in /www/vhosts/forums.int.com/forums/includes/class_bootstrap.php(433) : eval()'d code on line 146
[Tue Sep 26 00:54:18 2017] [error] [client 184.65.124.41] PHP Stack trace:
vbulletin, upgrading, fatal, php, errorsthe, incompatible, template, currently, deleted, templates, plugins, compatible, ll, disable, enable, tue, sep, stack, www, vhosts, forums, int, index, require_once, forum, eval, global, fetch_template, vb_bootstrap, process_templates, includes, functions, query_first, null, class_bootstrap,