Automator – How to know what I can do with Automator in each app

You can create services like this:

Then give the services shortcuts from System Preferences > Keyboard > Shortcuts > Services.

I prefer using third party applications instead of services to assign shortcuts to scripts though. There has been a bug since 10.7 where the shortcuts for services don't always work until you hover over the services menu from the menu bar (or for example run a service once from the menu bar). There is also a noticeable delay before services are run, and the shortcuts for services don't work when an application that doesn't have a services menu (like Tower) is frontmost.

More examples:

tell application "VLC"
    if playing then
        tell application "System Events" to tell process "VLC"
            click menu item "Pause" of menu 1 of menu bar item "Playback" of menu bar 1
        end tell
    else
        play
    end if
end tell

tell application "VLC"
    set n to 60
    if duration of current item - current time > n then
        set current time to current time + n
    else
        next
    end if
end tell

tell application "VLC"
    set fullscreen mode to not fullscreen mode
end tell

tell application "VLC"
    volumeUp
end tell

Open VLC's dictionary in AppleScript Editor to see all supported terminology.

If you use the Spotify desktop app, the media keys on OS X work by default (sometimes iTunes randomly "retains" the keys: just reboot). For the best results, don't keep iTunes and Spotify open at the same time.

If you use the Spotify Web Player, you can enable the media keys using BeardedSpice, a free application which enables you to control web-based media players using you keyboard media keys. It also works for YouTube, for example.