Hello,
If you don't need the customer to have access to the vendor shipping tracking number "automatically" ; the idea would be to open the custom field edition to the vendor even if the value will be stored only in the vendor sub-order.
It means that, if you want the customer to know the tracking, the administrator would need to copy the values from the sub-orders to the customer order.
It would require some development but it will be a smaller development than a full system.
Otherwise, it would require the special custom field type development, like I explained previously.
The HikaMarket feature in the TODO list is fully related on the HikaShop partial shipping and the fact that tracking number should be stored in the "order shipping params" (and not a custom field) so it will be supported by all components, plugins and views.
Some specifications has been written some time ago but they are not completed since we wanted to find a good way to be able to provide links for the customer without the seller/admin/vendor need to enter a full URL. There was also some specifications regarding partial shipping and a way to regroup several products in an order to mark them "shipped" or "refunded" ; the idea was to be able to provide more details, such as the tracking number when "shipping" so you could have information of multiple tracking with each time the associated products.
But as you will understand, it would require some important development, with various modification within HikaShop and its views and afterwards the modifications in HikaMarket to support the feature.
Regards,