In order to allow your customers to log in to your X-Cart store via Vkontakte social network, you should Register in VKontakte Developer portal
And open up "My apps" page.
Click on the "Crate an Application" button.
Select proper application type like on the next screenshot.
Fill in application data with info about your site.
Confirm the phone number connected with VKontakte account.
Fill in required information about your site. This information and logo will be displayed to customers, when they try to log in to X-Cart store via VKontakte.
On this step, wright down the required Application ID and Secure key.