The USER, VENDOR, PASSWORD are credentials for your Paypal Payments Pro UK account.
The USER and VENDER will be the merchant login name that you use to log into to manage your PayPal Payments Pro UK account.
The PWD will be assigned to you when you sign up for PayPal Payments Pro UK, it cannot be retrieved online from your account, so if you are not sure what to use, you will need to call paypal.