That's happened because if you rewrite some method of parent class it should contain the same amount of entered parameters and parameters should be the same type. In your case _beforeSave has one input parameter and its type is Mage_Core_Model_Abstract but you use Varien_Object. PHP doesn't allow you to do that. So instead of

protected function _beforeSave(Varien_Object $vendor)

Use

protected function _beforeSave(Mage_Core_Model_Abstract $object)

That's happened because if you rewrite some method of parent class it should contain the same amount of entered parameters and parameters should be the same type. In your case _beforeSave has one input parameter and its type is Mage_Core_Model_Abstract but you use Varien_Object. PHP doesn't allow you to do that. So instead of

protected function _beforeSave(Varien_Object $vendor)

Use

protected function _beforeSave(Mage_Core_Model_Abstract $vendor)