[Vitaly]

Looked into class_mail.php

It contains such strings:

        $fromemail = empty($vbulletin->options['bounceemail']) ? $vbulletin->options['webmasteremail'] : $vbulletin->options['bounceemail'];
        $headers .= 'Return-Path: ' . $fromemail . $delimiter;

So, it's not a problem to set right "Return-Path" (set $vbulletin->options['bounceemail'] before calling vbmail, and then restore back). "FROM" should be left intact (webmaster/admin email), to avoid trubles with spam protection.

The email selection algorithm:

1. If email field exist & filled in form - then take from there.
2. If not - then from user profile.