The following fields have been added to your media campaign and invoice and their line items:

Navigate to Custom Settings and click on manage for VAT. Open the relevant record combination of seller, and buyer and type and add an external name/id: 

Additionally you have to had a record for the VAT Exempt if you like to use an external name/id for the exempt value also:

The system then automatically populates the following fields on 

MediaCampaign/Invoice: 

Campaign Item and Invoice Item: