Adjusting screen brightness based on application
I'm using a little menubar program called ControlPlane.

It allows you to create "contexts". You define what evidence sources the program keeps an eye on, and when it detects the activation of some evidence source, it switches to a context and performs a set of actions that you've set it to do. It has a lot of actions to choose from, but if you don't find what you like, you can also set it to run an applescript that you've written, just remember to save the script as an application, for otherwise it will simply open the script as a text in Script Editor. I think you could try something like this:


Create a context and call it "video".
Set the activating trigger for the context to be, when it detects that VLC is frontmost. (Haven't looked but there might be further settings for using fullscreen as the trigger). - Then, add an action to the trigger: On arrival... to the "Video" context.... , no delay.
Create another trigger so that when ControlPlane detects you're no longer in that context (because the evidence source such as a specific open application has become false), Create an action "On departure... to the "Video" context... And set the brightness value back to normal.


I use ControlPlane in concert with a key binding program called BetterTouchTool.

If I wanted to have ControlPlane change the brightness level not whenever VLC is open, but whenever it's in fullscreen, I might first create the ControlPlane brightness action thing described above but set it to never look for evidence and change context spontaneously on its own. Instead, I could write an AppleScript which would command ControlPlane to toggle that context.

In BetterTouchTool, I could then select VLC as the application in question and create a new keyboard shortcut for it to override what it originally has. I think  + F normally toggles fullscreen mode in VLC. In BetterTouchTool, you could assign  + F to do multiple actions of your choice:


Toggle fullscreen AND run the applescript application which commands ControlPlane to change brightness.
Hit +F again with VLC foremost, and it would toggle that action set again.


BetterTouchTool also has an action to change the display brightness so you can also just skip ControlPlane, but I explained the whole ControlPlane thing in case that you actually do want to automate something for some apps without having to press any keys yourself.

I'm assuming that you're on a laptop or an iMac, are you? Because these brightness commands don't work for external monitors.