How to Get Raw Request Body in Magento 2 Controller

Finally i have resolve my issue . Only Need to override Subscriber. I am posting my entire code. may be it will helpful to someone. di.xml

<config xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:ObjectManager/etc/config.xsd">
    <preference for="Magento\Newsletter\Model\Subscriber" type="Navin\Subscribe\Model\Subscriber" />
</config>

Subscriber.php Model Files

namespace Navin\Subscribe\Model;

class Subscriber extends \Magento\Newsletter\Model\Subscriber {

    /**
     * Initialize resource model
     *
     * @return void
     */

    public function subscribe($email, $subscriber_name = '', $subscriber_date_of_birth = '', $subscriber_country_code = '') {

        $this->loadByEmail($email);

        if (!$this->getId()) {
            $this->setSubscriberConfirmCode($this->randomSequence());
        }

        $isConfirmNeed = $this->_scopeConfig->getValue(
                        self::XML_PATH_CONFIRMATION_FLAG, \Magento\Store\Model\ScopeInterface::SCOPE_STORE
                ) == 1 ? true : false;
        $isOwnSubscribes = false;

        $isSubscribeOwnEmail = $this->_customerSession->isLoggedIn() && $this->_customerSession->getCustomerDataObject()->getEmail() == $email;

        if (!$this->getId() || $this->getStatus() == self::STATUS_UNSUBSCRIBED || $this->getStatus() == self::STATUS_NOT_ACTIVE
        ) {
            if ($isConfirmNeed === true) {
                // if user subscribes own login email - confirmation is not needed
                $isOwnSubscribes = $isSubscribeOwnEmail;
                if ($isOwnSubscribes == true) {
                    $this->setStatus(self::STATUS_SUBSCRIBED);
                } else {
                    $this->setStatus(self::STATUS_NOT_ACTIVE);
                }
            } else {
                $this->setStatus(self::STATUS_SUBSCRIBED);
            }

            $this->setSubscriberEmail($_POST['email']);

        }

        if(!empty($_POST['subscriber_name']) ){
                $this->setSubscriberName($_POST['subscriber_name']); //subscriber_name
                $this->setSubscriberDateofbirth($_POST['subscriber_dateofbirth']); //date of birth
                $this->setSubscriberCountrycode($_POST['subscriber_countrycode']); //country code
        }

        if ($isSubscribeOwnEmail) {
            try {
                $customer = $this->customerRepository->getById($this->_customerSession->getCustomerId());
                $this->setStoreId($customer->getStoreId());
                $this->setCustomerId($customer->getId());
            } catch (NoSuchEntityException $e) {
                $this->setStoreId($this->_storeManager->getStore()->getId());
                $this->setCustomerId(0);
            }
        } else {
            $this->setStoreId($this->_storeManager->getStore()->getId());
            $this->setCustomerId(0);
        }

        $this->setStatusChanged(true);

        try {
            $this->save();
            if ($isConfirmNeed === true && $isOwnSubscribes === false
            ) {
                $this->sendConfirmationRequestEmail();
            } else {
                $this->sendConfirmationSuccessEmail();
            }
            return $this->getStatus();
        } catch (\Exception $e) {
            throw new \Exception($e->getMessage());
        }
    }

}

Your code is working perfectly fine for me. The issue must be with no value for that attribute with the particular customer_is. Have you tried and saved is_vendor value for customer_id 5 once. I mean just try and change the value of is_vendor attribute and save that customer. Now execute your code, i hope you should get the value. If you have not followed above process then you will get empty array because that attribute did not have any value set for those customer id in database.

This is not an issue I must say, magento always work like that only, if you will create a new customer then things will work fine because customer form has that attribute and it will save its default value but already created customer did not have that attribute known to them, thus you need to write a small script to save all customer that were created previously and set whatever value you want for them.(this will be just one time process).

Below code will do the work:-

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$customerObj = $objectManager->create('Magento\Customer\Model\Customer')->getCollection();
foreach ($customerObj as $customerObjdata) {
    $customermodel  = $objectManager->create('Magento\Customer\Model\Customer');
    $customerData = $customermodel->getDataModel();
    $customerData->setId($customerObjdata->getData('entity_id'));
    $customerData->setCustomAttribute('is_vendor', 0);
    $customermodel->updateData($customerData);
            
    $customerResource = $objectManager->create('\Magento\Customer\Model\ResourceModel\CustomerFactory')->create();
    $customerResource->saveAttribute($customermodel, 'is_vendor');
}