The following is what you'll need to setup Vtiger CRM in Formstack Sync.
Your Vtiger URL: This what you see in your browser when you are logging in to your instance of Vtiger CRM. You do not need index.php or anything thereafter.
Username and Password: This is simply taken from a valid user of your instance of Vtiger CRM.
The Users Access Key: Find the Access Key for this user by clicking on the settings icon in the upper right-hand corner when you are logged in to your Vtiger account and click on CRM Settings. Select User & Access Control on the left-hand side then select Users. Click on the User you are using to authenticate the integration and scroll to the bottom under the Advanced Options and here you'll find the Access Key.