Workaround to VichUploaderBundle's PRE persist event instead of POST persist, which prevents from accessing / using the entity's generated ID to name the folder or file. Please note that this is not working code but an extract that needs to be adapted for your needs / implementation !

CustomDirectoryNamer.php

<?php
class CustomDirectoryNamer implements Vich\UploaderBundle\Naming\DirectoryNamerInterface
{
    public function directoryName($object, PropertyMapping $mapping): string
    {
        if (!$object instanceof HasUploadedFile) {
            throw new \UnexpectedValueException(
                sprintf(
                    'The class "%s" only supports objects of type "%s".',
                    self::class,
                    HasUploadedFile::class
                )
            );
        }

        // Returns the ID as the folder's name
        return (string) (new \Symfony\Component\PropertyAccess\PropertyAccessor())
            ->getValue($object, $object->getIdPropertyName());
    }
}

CustomEntityManager.php

<?php
class CustomEntityManager
{
     /**
     * VichUploaderBundle is plugged on Doctrine's PRE_PERSIST event. Which means that we can't access the entity ID
     * to generate the folder name, by design.
     *
     * @see https://github.com/dustin10/VichUploaderBundle/issues/473#issuecomment-158808635
     *
     * As a workaround - which has its drawbacks, notably on performance as we persist the object twice -, when the
     * entity is first created AND there is a file attached to upload, we store the file on the side (so not to trigger
     * VichUploaderBundle), save the entity (to generate its ID), then trigger the upload manually (so we can use the ID
     * to generate the folder's name), and finally save the entity again (as the filename may have changed and it needs
     * to be updated in the database too).
     */
    public function saveUploadableEntity(
        HasUploadedFile $entity,
        CustomEntityRepository $repository,
        \Vich\UploaderBundle\Handler\UploadHandler $vichUploaderManager,
        bool $flush = true
    ): void {
        $propertyAccessor = new \Symfony\Component\PropertyAccess\PropertyAccessor();

        $idPropertyName = $entity->getIdPropertyName();
        $filePropertyName = $entity->getUploadedFilePropertyName();

        $fileToUpload = $propertyAccessor->getValue($entity, $filePropertyName);
        $hasFileToUploadOnCreation = (
            null === $propertyAccessor->getValue($entity, $idPropertyName) && // new object
            null !== $fileToUpload // has a file to upload
        );

        if ($hasFileToUploadOnCreation) {
            $propertyAccessor->setValue($entity, $filePropertyName, null);
        }

        $repository->save(
            $entity,
            $hasFileToUploadOnCreation
                ? true // here we need to flush if we want to have access to the generated ID
                : $flush
        );

        if ($hasFileToUploadOnCreation) {
            $propertyAccessor->setValue($entity, $filePropertyName, $fileToUpload);
            $vichUploaderManager->upload($entity, $entity->getUploadedFilePropertyName());
            $repository->save($entity, $flush);
        }
    } 
}

CustomEntityRepository.php

<?php
class CustomEntityRepository extends Doctrine\Bundle\DoctrineBundle\Repository\ServiceEntityRepository
{
    public function save(object $entity, bool $flush = true): void
    {
        $this->_em->persist($entity);
        if ($flush) {
            $this->_em->flush();
        }
    }
}

Entity.php

<?php
class Entity implements HasUploadedFile
{
    /**
     * @ORM\Column(name="id", type="integer")
     * @ORM\Id
     * @ORM\GeneratedValue(strategy="AUTO")
     */
    public $id;
  
    /**
     * @Vich\UploadableField(mapping="{XXXXXXXXXXXXXXX}", fileNameProperty="fileName")
     */
    public $file;
  
    /**
     * @ORM\Column(name="fileName", type="string", nullable=true, length=255)
     */
    public $fileName;
  
    public function getIdPropertyName(): string
    {
        return 'id';
    }

    public function getUploadedFilePropertyName(): string
    {
        return 'file';
    }
}

HasUploadedFile.php

<?php
interface HasUploadedFile
{
    public function getIdPropertyName(): string;

    public function getUploadedFilePropertyName(): string;
}