If its for a postnuke page, you might find including vbulletin's global file will mess up PHP's enviroment and stop it from working. I cant imagine it doing any real damage anymore since most things are stored inside classes.
You might have to chdir back to the directory your file is located in otherwise it wont be able to require the extra files that are required at the bottom of the code you pasted.
I tried to include the global.php file inside phpnuke just to test it out and chdir back but it DID break the enviroment like you predicted Merk!