Many thanks indeed!
I see something interesting here. The code has referrences to a php variable and the javascript function mhash() in the onsubmit, which aren't in the html page (or included). Yet it still works. In fact, I removed all references to the function and php variable, and it *still* works! Perhaps the vBulletin login.php page sees missing values and generates them at that point.
This is the html code, amazingly:
<form action="path-to-your-forum/forums/login.php?do=login" method="post">
<input type="hidden" name="do" value="login" />
<input type="hidden" name="url" value="path-back-to-your-original-page" />
<input type="hidden" name="vb_login_md5password" />
<input type="hidden" name="vb_login_md5password_utf" />
<input type="hidden" name="s" value="" />
<input type="hidden" name="cookieuser" value="1" id="cb_cookieuser" tabindex="1" />
<label>Username:<input name="vb_login_username" type="text" id="login" tabindex="1" class="bginput" accesskey="u"/></label>
<label>Password<input name="vb_login_password" type="password" id="password" tabindex="1" class="bginput" /></label>
<input name="submit" type="submit" id="submit" tabindex="1" value="Login" accesskey="s" class="button" />
</form>
! Cheers, Jeffrey