Honestly Fonzi… YES… the benefit absolutely outweighs the cost. Using an ESP to communicate to your forum members is way easier than using vBulletin. Plus, vBulletin’s delivery is quite poor and they don’t provide any reporting metrics, so there’s really no way to measure what’s working and what’s not.
As for subscribing/unsubscribing… Users will be able to do that through the ESP, so you can manage all email correspondence that way. How come you would want that info to carry over to vBulletin as well?