My honest evaluation...
I would not bolt on vbulletin.
This modification WILL come to pass. And it's very close to being stable enough for a heavy roll out.
Time table? I can't be certain. I'd love to have this rolling out in the next two weeks.
In fact I'm taking next monday off my paying job (using a vacation day) to dump a significant amount of time into this modification to try to finish resolving a few of the problems remaining.
The current version her eon vborg, is "okay". Sometimes the parsing is crappy and it'll show extra = signs, and attachments aren't handled very well.
The version I have built so far is pretty solid.
I can promise a new version in 2 weeks that may not use vbmail.
However, I'm hoping to get one that uses vbmail by that same time frame.
I know this isn't the definitive concrete answer you were hoping for.. but it's an honest one