[Muellmann]

The solution would be an example like this:
(easy learning by doing - this is not a ready product just to install ):

1) Go in your Admin Control Panel (ACP)
2) Scroll down to "Plugins & Products" on the left side in acp's nav-frame
3) Expand "Plugin & Products" and choose "Add New Plugin"
4) Then specify:

Product: vBulletin (if you haven't till yet an own product specified, so keep "vBulletin" as default)
Hook Location: reputation_add_process (this is the hook on which your code runs)
Title: inform members about received reputation (or whatever you want)
Execution order: 5 (leave the default if you haven't many other plugins on the same hook)
Plugin PHP Code*: (it's the php-code that will run on this hook)

mail($userinfo['email'], "Reputation with score ".$score." was given at post no. ".$postid." by ".$vbulletin->userinfo['username'], "reason:\n".$vbulletin->GPC['reason']."\nthread title:\n".$threadinfo[title], "From: ".$vbulletin->options['webmasteremail']."\r\n");

Plugin is Active: Choose "YES" or it won't work! (no is the default)

5) Control if all inputs are matching and then click the "SAVE" button below!

You're done and you can be proud! You made your first little plugin

*This is just a very simple sample. You can modify it by your own wishes if you know what you do .
Note: If comes a white page instead of the page redirect
your own php code could contain a parse error.