[Amaca]

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:

<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.