Vanilla 2 incorporates this as a standard feature. Simply edit your preferences from your profile page, and set vanilla to notify you when people comment on your discussions.
Not via a UI yet, but you can set the default prefs for new users by editing your config file. The following settings exist in config-defaults.php and you can copy them into your config.php file to override the defaults:
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Vanilla Forums COO [GitHub, Twitter, About.me]
Vanilla Forums COO [GitHub, Twitter, About.me]