I had a quick look at your code, unless I'm reading it wrong I think it's already covered by vBMail
The hook you used is executed in
exec_send_notification, and the query only returns users that match
Code:
subscribethread.emailupdate IN (1, 4)
whereas part of vBMail's "remove the user from everything email related" cron job code is setting that column to 0 - so in theory it shouldn't fetch users vBMail has dealt with
For the record; we'll never give permission to re-use our code
Fillip