I've managed to get google translation working on my vBulletin site. I placed the script in the welcome headers section for guests and in the log in section for members. It seems to work. Just place code where you want it in your forum and it translates all the pages.
Script I got from
here and I modified it to work on my site by centering the script. The only other thing to do is point the script to the image ("Translate this page by google"), which you can get from the script site.
I'd love to use flags so if anyone knows how to modify the script to show flags instead of a drop-down menu (which is in English.), please post.
You can see the translation in action at my site
here.
If the translation produces junk then post here for others to know.
NOTE: Translation button now centered in header section.