It’s possible that Vanilla Forums was updated and now requires InnoDB.
Taking a quick look at their website, it seems they released a 4.0 version last week. In the announcement topic they say it requires MySQL 5.7 or higher, but we still run MySQL 5.6.
So it seems you can’t run Vanilla Forums 4 here. Please try install Vanilla Forums version 3 instead.
What @Admin said but, when and if InfinityFree upgrades the MySQL version, and if InnoDB won’t still be available , you can force the use of MyISAM adding this to your /conf/config.php file: