PayPal 
1Log in to your PayPal account
2Click on the Profile subtab
3Click on the Instant Payment Notification Preferences link in the 'Selling Preferences' column
4Click Edit
You should now be presented with a screen containing the following:

Setting callback URL

Note:
If the Instant Payment Notification link is not present you will need to apply for Premier Account or a Business Account from PayPal.
Check the checkbox to activate Instant Payment Notification and enter the following URL below the checkbox.

http://www.example.com/forums/subscriptions/paypal.php

Click [Save]

Go to vBulletin Options > vBulletin Options > Paid Subscriptions (link) and enter your PayPal email address in the PayPal email field.
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