Hi,
Your offer appears reasonable. I don't consider it spamming, and I appreciate you offering the hackers here free membership to your support forums. That's generous of you. Most hackers here have found that they can do a lot of what vBPortal does with minor hacking involved without the need for so much extra code. Of course, we don't have the functionality and ease of the addition of vBPortal modules (ported from Nuke PHP).
However, you do have stiff competition, from free portal hacks, to commercial ones like Tingle, and also one by Firefly, all of which appear to be less server-intensive than vBPortal.