Problem/Motivation
This could possibly be classified as a bug report. The entity annotation for the Vocabulary class sets the overview form to Drupal\taxonomy\Form\OverviewTerms. Ideally this can be overridden by calling \Drupal\Core\Entity\EntityType::setFormClass from hook_entity_type_alter, however it has no effect since VocabularyRouteProvider::getOverviewPageRoute hardcodes the form class.
Steps to reproduce
Proposed resolution
Update \Drupal\taxonomy\Entity\Routing\VocabularyRouteProvider::getOverviewPageRoute to call $route->setDefault('_form', $entity_type->getFormClass('overview'));.
Remaining tasks
Test coverage