With this VirtueMart plugin, all buyers of a product can be automatically added to an AcyMailing list upon purchase. Additionally (or alternatively), a "Subscribe" and "Unsubscribe" button can be shown on the product page to allow manual subscription and unsubscription of the product-specific mailinglist.

Typical use cases are:
-) Automatically subscribe buyers to newsletter about updates to the products they bought
-) Sell subscription to newsletters (by disallowing manual subscription, so only buyers can ever be subscribed)
-) Let users unsubscribe from product-spedific newsletters

For each product, you can select:

-) The AcyMailing list for the product (multiple lists are possible), multiple products can also have the same list attached
-) Whether buyers will automatically subscribed upon purchase
-) Whether the "Subscribe"/"Unsubscribe" buttons are shown for registered or anonymous users, or not at all.

This plugin also needs to AcyMailing component installed (which can be downloaded for free).