Password?!? vBulletin 4 stores its passwords encrypted with MD5 and a salt. (Although a very bad hashing algorithm.)
I believe the bbuserinfo array can be used in that template. Then it's just a matter of doing this in the subscription_payment_ccbill template.
PHP Code:
<input type="hidden" name="clientAccnum" value="{vb:raw settings.clientAccnum}" />
<input type="hidden" name="clientSubacc" value="{vb:raw settings.clientSubacc}" />
<input type="hidden" name="formName" value="{vb:raw settings.formName}" />
<input type="hidden" name="subscriptionTypeId" value="{vb:raw subinfo.ccbillsubid}" />
<input type="hidden" name="allowedTypes" value="{vb:raw subinfo.ccbillsubid}" />
<input type="hidden" name="email" value="{vb:raw settings.email}" />
<input type="hidden" name="hash" value="{vb:raw hash}" />
<input type="hidden" name="username" value="{vb:raw bbuserinfo.username}" />
<input type="hidden" name="password" value="{vb:raw bbuserinfo.password}" />
<input type="hidden" name="noCache" value="1" />