Solve it
it turns out that X-Cart handles orders in Multi-Vendor as Children, where each child represents a vendor. Therefore, to access methods we need first to specify the child (vendor).
So instead of:
PHP Code:
$this->getShippingMethodName();
We need to use:
PHP Code:
$this->getChildren()[0]->getShippingMethodName();
Where [0] => first child (vendor) of the order.