I guess what's happening is that access through bridge.php is causing the exitsing session to be deleted, but if you have a cookie from checking that box, then it will log you back in. I don't know why the session would end though.

global $vbulletin;
global $bootstrap;

not work... logged out my account :(
global $vbulletin; work only without www.

The first code you posted all works OK, right? (The one that's not in a function). Even if you don't check guardar? If that's true, then I think the problem must be that there's some variable that needs to be global.

Edit: now that I look at the code a little more, it looks like the only variables that get created in a place that assumes it's not within a function are $bootstrap and $permissions. You need "global $vbulletin" so that you can use the variable in your function, but it doesn't have to be before the include. So you can try adding $permissions to the list, but if that doesn't work, but the code works outside any function, then I'm not sure what's going on. You might have to look at the code in class_core.php that creates a session and see if you can figure out why your bridge.php script is not getting associate with the existing session (I believe it should be getting the sessionid from a cookie).