Magento 1.9 – Fix Custom Product Attribute is Null

I use Magento 1.9.2.1.

I've created a custom attribute and now I've been trying to get it in the cart page (template/checkout/cart.phtml:146-148) in this way

Zend_Debug::dump($_item->getProduct()->getData())

or

Zend_Debug::dump($_item->getData())

The key exists but the value is null.

I've looked and followed all steps at this link:

http://www.atwix.com/magento/custom-product-attribute-quote-order-item/

I've already cleaned cache and reindex all indexes.

This is the install script

$installer = $this;
$installer->startSetup();

$installer->addAttribute('catalog_product', 'vendor_id', array(
    'group'                     => 'General',
    'type'                      => 'int',
    'backend'                   => '',
    'frontend'                  => '',
    'label'                     => 'Vendor',
    'input'                     => 'select',
    'class'                     => '',
    'source'                    => 'mynamespace_mymodule/source_vendor',
    'global'                    => Mage_Catalog_Model_Resource_Eav_Attribute::SCOPE_STORE,
    'visible'                   => true,
    'required'                  => true,
    'user_defined'              => true,
    'default'                   => '',
    'searchable'                => true,
    'filterable'                => true,
    'comparable'                => true,
    'visible_on_front'          => true,
    'unique'                    => false,
    'apply_to'                  => 'simple',
    'is_configurable'           => false,
    'used_in_product_listing'   => true,
));
$installer->endSetup();

This is the upgrade script

$installer = new Mage_Sales_Model_Resource_Setup('core_setup');

$entities = array(
    'quote',
    'quote_address',
    'quote_item',
    'quote_address_item',
    'order',
    'order_item',
);
$options = array(
    'type'     => 'int',
    'visible'  => true,
    'required' => true
);
foreach ($entities as $entity) {
    $installer->addAttribute($entity, 'vendor_id', $options);
}
$installer->endSetup();

This is my config.xml

    <sales>
        <quote>
            <item>
                <product_attributes>
                    <vendor_id />
                </product_attributes>
            </item>
        </quote>
    </sales>

    <fieldsets>
        <sales_convert_quote_item>
            <vendor_id>
                <to_order_item>*</to_order_item>
            </vendor_id>
        </sales_convert_quote_item>
        <sales_convert_order_item>
            <vendor_id>
                <to_quote_item>*</to_quote_item>
            </vendor_id>
        </sales_convert_order_item>
    </fieldsets>

    <events>
        <sales_quote_item_set_product>
            <observers>
                <mynamespace_mymodule>
                    <class>mynamespace_mymodule/observer</class>
                    <method>salesQuoteItemSetVendorId</method>
                </mynamespace_mymodule>
            </observers>
        </sales_quote_item_set_product>
    </events>

And this is the observer

class Mynamespace_Mymodule_Model_Observer extends Varien_Object
{
    public function salesQuoteItemSetVendorId($observer)
    {
        $quoteItem = $observer->getQuoteItem();
        $product = $observer->getProduct();
        $quoteItem->setVendorId($product->getVendorId());

        return $this;
    }
}

On database side the attribute exists in eav_attribute and is assigned to two product in catalog_product_entity_int.

I don't know what to do to get it work!

Someone please suggest me some tests to find out why it does not work.

UPDATE 2015-11-04

If I print out the product data inside the observer file, the vendor_id is not present

Zend_Debug::dump($observer->getProduct()->getData());

Se now the question is why my custom attribute is missing in the product?