I have fixed it changing the code in the vBulletin HEADER file with the same code of the vBulletin Welcome block Widget for wordpress.
go to the vBulletin admin cp -> style manager -> edi template -> header
and search the line <form id="navbar_loginform" ecc.. Select all the form and replace the code with this:
Code:
<form id="navbar_loginform" action="http://yourwebsitename/forum/login.php" method=post onsubmit=md5hash(vb_login_password,vb_login_md5password,vb_login_md5password_utf)>
<script type=text/javascript src="http://yourwebsitename/forum/clientscript/vbulletin_md5.js"></script>
<fieldset id="logindetails" class="logindetails">
<div>
<div>
<input class="textbox<vb:if condition="!$username"> default-value</vb:if>" name="vb_login_username" id="navbar_username" size="10" accesskey="u" tabindex="101" value="<vb:if condition="$username">{vb:raw username}<vb:else />{vb:rawphrase username}</vb:if>" />
<input type="password" class="textbox" tabindex="102" name="vb_login_password" id="navbar_password" size="10" />
<input type="text" class="textbox default-value" tabindex="102" name="vb_login_password_hint" id="navbar_password_hint" size="10" value="{vb:rawphrase password}" style="display:none;" />
<input type=submit class="loginbutton" tabindex="104" value="{vb:rawphrase log_in}" title="{vb:rawphrase enter_username_to_login_or_register}" accesskey="s"/>
</div>
</div>
</fieldset>
<div id="remember" class="remember">
<label for=cb_cookieuser_navbar>Remember me</label>
<input name=cookieuser type=checkbox id=cb_cookieuser_navbar value=1 checked=checked />
</div>
<input type=hidden name=s value= />
<input type=hidden name=do value=login />
<input type=hidden name=vb_login_md5password />
<input type=hidden name=vb_login_md5password_utf />
</form>
Remember to change the URL in the first line.
it works for me, hope it helps.