Data modelling best practices

Are there any data modelling best practices?

Let's say I am creating a master detail SAPUI5 app to display purchase orders and their items.

So I have got two entities PurchaseOrder (header) and PurchaseOrderItem and an association + navigation between them and that concept is clear.

But also for my PurchaseOrder entity I would like to show the vendor name. As we know table EKKO has only LIFNR (vendor number). Should I model my PurchaseOrder entity that it has VendorName in its properties and in the GET_ENTITY and GET_ENTITYSET join EKKO with LFA1 table to get the vendor name. Or should I create another Vendor entity, create a navigation between PurchaserOrder and Vendor entity and in my SAPUI5 app use the 'expand' parameter to fetch the vendor name? Which is the correct approach and why? When should I create a new entity with navigation and when should I fetch the additional data in the existing entity?

Thanks,

Radek