Here is something you can try to make sure that you receive the vouchers in the future:
Login to your Plugin Alliance account. Click on 'My Account'. Scroll down and click on Update Email Preferences tab at the bottom of the page under the User Profile heading. Once there, make sure that the 'Sales & Special Promotions' box is ticked.
If you ever click 'Unsubscribe' at the bottom of any of our automated newsletters, you are taken out of all the categories, including the one that is responsible for the Customer Loyalty Vouchers.