Excellent question! The billing country may be limited in 2 ways. First, you can set the accepted billing countries for the Visa Checkout lightbox by going into your Visa Checkout developer dashboard and opening the project. Click on Configuration in the menu. Select the dropdown next to Visa Checkout and click to open Checkout Profiles. Click the plus sign / 'create new' button. Give your new profile a name and select which billing countries you would like to accept. Save your changes. Add the profile ID to your v.init event handler.
The second option is overriding the default billing countries by adding the billingCountries accepted list directly to the v.init event handler. You can see an example of this on pages 51-52 of the Visa Checkout Integration Guide.