Which style? Do you use the vbulletin standard style or one style from vbstyles?
You can change the template "navbar" for that. Look for
Code:
<if condition="$show['registerbutton']">
<td class="vbmenu_control"><a href="register.php$session[sessionurl_q]" rel="nofollow">$vbphrase[register]</a></td>
</if>
and change the url to your register page to your url from your x-cart register.php. Every time a user clicks on this link he will be redirected.
Then look for the section "<!-- login form -->" in the same template. These is the code for the two login boxes for the username and password. You can either remove this section completely (from <!-- login form --> to "<!-- / login form -->" or write your own code in these section.
I would prefer the use of the welcome headers from vbulletin.org because you can write your own welcome message with instructions how the users should register.