I think you underestimate the amount of time it takes vbulletin to process and deliver those emails
While I definately agree that putting it on a forum where you could have 1000 members autosubscribed to it would add a bit of a delay in the posts coming back, which is also why I strongly suggest not to use the autosubscription for large usergroups.
Also realize that the number of your users that will utilize this feature will not be all of them.
Alot of users HATE mailing lists. That's what this is...
However... alot of users HATE forums...
This let's you get those forum haters to participating in your forum and through the links in the bottom of the emails, gradually get them to roll around to the forum life
Please feel free to run this on a test server and try it out.
While the code is not yet at it's optimal form, it's definately strong enough to handle a significant load.