When the plugin is executed it will have the vBulletin installation directory as current directory. Including a file in another directory (include('../poll/currentpoll.php')

does not chdir() to that directory, so the included script will still be running "from" the vB directory. So if your script includes "poll.php" first the current working directory (ie. your vB installation directory) will be searched first for the a script with this name and it will find the vBulletin poll.php file.