Step by Step guide: How do I upgrade the OS on my BlackBerry? - BlackBerryFAQ
You will not lose the vodafone branding with any OS, that is hard coded in the device. You can retain your themes and settings fine. You could have a glitch here or there with a new OS, that is the gamble. But overall, you should not have a problem.
Bottom-line--if you are not comfortable doing this, don't.