OrderCentral calculates VAT based upon the country of the shipment address and the corresponding VAT Rule. If this is not per your requirements there is the option to write you own logic to calculate VAT. This article describes how to do this.
All referenced class names are within the welisacommerce namespace
Create a new Apex class and have it implement the ITaxCalculator interface
Implement your logic and return an instance of TaxCalculationResponse (all properties are required
Register your class within the OrderCentral plugin registrations
Setup -> Custom Metadata Types -> Plugin Registrations -> New
In the Plugin field select pluging with name ITaxCalculator