You have to use the patch program. diff is only for finding the difference (diff --help and patch --help should give some help text).

If you're unsure about the diff format, have a look at it with a text editor. It should be quite self-explaining if you try to apply the patches manually - that would also be the best if your installation of vBB is customised.

@freddie:
I personally don't run a vbb (I would prefer an open licensed one like phpBB if I had to make a decision), but my boss allows me to contribute the work done on the firm's board. If this is against the forum rules/license I will stop (just EMail me in this case) - as I only post diffs and own code (as public domain) this should be no problem in my eyes.

@The_Sisko:
A patch in unified diff format is basically a very schematic textual description of what has changed. Look at it with a text editor (which is capable of Un*x line endings - not notepad) and you will see that it contains exact information (quite self explaining) about the files affected, the changed lines and what has been altered.
In my eyes this is better than a "normal install text". Besides this I'm not interested in doing too much work for a proprietary product which I even don't have at home.