Hi Sync,
Ok it seems to be working.
However, I don't like the process, I mean go to checkout, go to CB, and need to go back once again in check out.
So I'll hack your hackĀ

or the easiest way would be to have the CB field required and the VM field in the same page as on VM when you don't use the auto completion password
moreover I guess we do have to change this line of code in virtuemart.php CB plugin also
DEFINE( '_VIRTUEMART_SECRET' , 'xxxx' );
in order to get our own secret key, right ?
I don't know how works the CB plugin, I do not have the VM field required, actually I filled the nickname, name, email, etc...
and log in to the website, and redo the checkout and here I see that VM don't ask for the email for example, so you're on the good road ! :-)
in order to make a good shop, not a geek place

we need:
1/ if the CB registration is on a page alone (has it is now), at least the registration process should avoid us to enter the nickname choosen and the password (I've already made this on phpshop and an old CB...), plus we need to have an automatic redirection to the checkout process from VM (all as to be hidden for the user)
2/if the CB registration field can be integrated in the checkout_register_form.php it would be better for the non geek understanding
3/ the better way would be to hack VM in order to populate CB with the new data just entered I'va also done that on phpshop...)
it's the better way in word of graphics integration and logical process for our costumers who're not 12 horus a day on their keyboard (like us I guess ;-) )
I guess I'll make the third option, let me know what you think ?
I love the way you use the plugin cause it's actually a very easy hack to implement inthe core for the future, but I don't like the checkout process, you understand ? (sorry for my english I'm french)