No problem! I just wish I could help you more (and now that you mention it, I do remember that you were installing your own custom php.ini).
I just went and took a look at that link for the vBulletin Mail System, and I'm wondering about the IMAP requirement. I know you did a custom php.ini, but did you do a complete recompile of PHP to add the imap extensions (more on that in this thread)? I'm thinking you will probably have to do that, but as vBulletin is a commercial product that I don't use, I can't test it for you to be sure.
Maybe someone here can share what they are doing with vBulletin and that hack.
I have installed other webmail systems that worked in a similar fashion on DreamHost (several years ago) and got them to work (using POP3), but quickly abandoned any thought of offering public "webmail" due to spam concerns and DH's implementation of user-based email quotas.
Depending upon how they are sent, DH allows 100 or 200 outgoing emails an hour, and that just won't work for most public webmail use.
I'm sorry I can't be of more help to you on this.